diff --git a/docs/Range.md b/docs/Range.md
index 909d0e3..644711e 100644
--- a/docs/Range.md
+++ b/docs/Range.md
@@ -45,10 +45,11 @@ are equal or within another, taking into account the end-inclusiveness:
     assert( (1..<3) in (1..3) )
     >>> void
 
-## Finite Ranges are iterable
+## Ranges are iterable
 
-So given a range with both ends, you can assume it is [Iterable]. This automatically let
-use finite ranges in loops and convert it to lists:
+Finite ranges are [Iterable] and can be used in loops and list conversions.
+Open-ended ranges are iterable only with an explicit `step`, and open-start
+ranges are never iterable.
 
     assert( [-2, -1, 0, 1] == (-2..1).toList() )
     >>> void
@@ -70,6 +71,26 @@ but
     >>> 2
     >>> void
 
+### Stepped ranges
+
+Use `step` to change the iteration increment. The range bounds still define membership,
+so iteration ends when the next value is no longer in the range.
+
+    assert( [1,3,5] == (1..5 step 2).toList() )
+    assert( [1,3] == (1..<5 step 2).toList() )
+    assert( ['a','c','e'] == ('a'..'e' step 2).toList() )
+    >>> void
+
+Real ranges require an explicit step:
+
+    assert( [0,0.25,0.5,0.75,1.0] == (0.0..1.0 step 0.25).toList() )
+    >>> void
+
+Open-ended ranges require an explicit step to iterate:
+
+    (0.. step 1).take(3).toList()
+    >>> [0,1,2]
+
 ## Character ranges
 
 You can use Char as both ends of the closed range:
@@ -98,6 +119,7 @@ Exclusive end char ranges are supported too:
 | isEndInclusive  | true for '..'                | Bool          |
 | isOpen          | at any end                   | Bool          |
 | isIntRange      | both start and end are Int   | Bool          |
+| step            | explicit iteration step      | Any?          |
 | start           |                              | Any?          |
 | end             |                              | Any?          |
 | size            | for finite ranges, see above | Long          |
@@ -105,4 +127,4 @@ Exclusive end char ranges are supported too:
 
 Ranges are also used with the `clamp(value, range)` function and the `value.clamp(range)` extension method to limit values within boundaries.
 
-[Iterable]: Iterable.md
\ No newline at end of file
+[Iterable]: Iterable.md
diff --git a/lyng-idea/src/main/kotlin/net/sergeych/lyng/idea/highlight/LyngLexer.kt b/lyng-idea/src/main/kotlin/net/sergeych/lyng/idea/highlight/LyngLexer.kt
index b2f02a7..bd14f57 100644
--- a/lyng-idea/src/main/kotlin/net/sergeych/lyng/idea/highlight/LyngLexer.kt
+++ b/lyng-idea/src/main/kotlin/net/sergeych/lyng/idea/highlight/LyngLexer.kt
@@ -36,7 +36,7 @@ class LyngLexer : LexerBase() {
         "abstract", "closed", "override", "static", "extern", "open", "private", "protected",
         "if", "else", "for", "while", "return", "true", "false", "null",
         "when", "in", "is", "break", "continue", "try", "catch", "finally",
-        "get", "set", "object", "enum", "init", "by", "property", "constructor"
+        "get", "set", "object", "enum", "init", "by", "step", "property", "constructor"
     )
 
     override fun start(buffer: CharSequence, startOffset: Int, endOffset: Int, initialState: Int) {
diff --git a/lynglib/src/commonMain/kotlin/net/sergeych/lyng/Compiler.kt b/lynglib/src/commonMain/kotlin/net/sergeych/lyng/Compiler.kt
index a4b9bfc..30c8aac 100644
--- a/lynglib/src/commonMain/kotlin/net/sergeych/lyng/Compiler.kt
+++ b/lynglib/src/commonMain/kotlin/net/sergeych/lyng/Compiler.kt
@@ -703,6 +703,10 @@ class Compiler(
 
     private suspend fun parseTypeAliasDeclaration(): Statement {
         val nameToken = cc.requireToken(Token.Type.ID, "type alias name expected")
+        val startPos = pendingDeclStart ?: nameToken.pos
+        val doc = pendingDeclDoc ?: consumePendingDoc()
+        pendingDeclDoc = null
+        pendingDeclStart = null
         val declaredName = nameToken.value
         val outerClassName = currentEnclosingClassName()
         val qualifiedName = if (outerClassName != null) "$outerClassName.$declaredName" else declaredName
@@ -719,13 +723,13 @@ class Compiler(
         }
         val typeParamNames = uniqueParams
         if (typeParamNames.isNotEmpty()) pendingTypeParamStack.add(typeParamNames)
-        val body = try {
+        val (body, bodyMini) = try {
             cc.skipWsTokens()
             val eq = cc.nextNonWhitespace()
             if (eq.type != Token.Type.ASSIGN) {
                 throw ScriptError(eq.pos, "type alias $qualifiedName expects '='")
             }
-            parseTypeExpressionWithMini().first
+            parseTypeExpressionWithMini()
         } finally {
             if (typeParamNames.isNotEmpty()) pendingTypeParamStack.removeLast()
         }
@@ -738,7 +742,16 @@ class Compiler(
             outerCtx?.classScopeMembers?.add(declaredName)
             registerClassScopeMember(outerClassName, declaredName)
         }
-        pendingDeclDoc = null
+        miniSink?.onTypeAliasDecl(
+            MiniTypeAliasDecl(
+                range = MiniRange(startPos, cc.currentPos()),
+                name = declaredName,
+                typeParams = typeParams.map { it.name },
+                target = bodyMini,
+                doc = doc,
+                nameStart = nameToken.pos
+            )
+        )
 
         val aliasExpr = net.sergeych.lyng.obj.TypeDeclRef(body, nameToken.pos)
         val initStmt = ExpressionStatement(aliasExpr, nameToken.pos)
@@ -2422,7 +2435,10 @@ class Compiler(
                     // to skip in parseExpression:
                     val current = cc.current()
                     val right =
-                        if (current.type == Token.Type.NEWLINE || current.type == Token.Type.SINGLE_LINE_COMMENT)
+                        if (current.type == Token.Type.NEWLINE ||
+                            current.type == Token.Type.SINGLE_LINE_COMMENT ||
+                            current.type == Token.Type.STEP
+                        )
                             null
                         else
                             parseExpression()
@@ -2440,6 +2456,30 @@ class Compiler(
                     }
                 }
 
+                Token.Type.STEP -> {
+                    val left = operand ?: throw ScriptError(t.pos, "step requires a range")
+                    val rangeRef = when (left) {
+                        is RangeRef -> left
+                        is ConstRef -> {
+                            val range = left.constValue as? ObjRange ?: run {
+                                cc.previous()
+                                return operand
+                            }
+                            val leftRef = range.start?.takeUnless { it.isNull }?.let { ConstRef(it.asReadonly) }
+                            val rightRef = range.end?.takeUnless { it.isNull }?.let { ConstRef(it.asReadonly) }
+                            RangeRef(leftRef, rightRef, range.isEndInclusive)
+                        }
+                        else -> {
+                            cc.previous()
+                            return operand
+                        }
+                    }
+                    if (rangeRef.step != null) throw ScriptError(t.pos, "step is already specified for this range")
+                    val stepExpr = parseExpression() ?: throw ScriptError(t.pos, "Expected step expression")
+                    val stepRef = StatementRef(stepExpr)
+                    operand = RangeRef(rangeRef.left, rangeRef.right, rangeRef.isEndInclusive, stepRef)
+                }
+
                 Token.Type.LBRACE, Token.Type.NULL_COALESCE_BLOCKINVOKE -> {
                     operand = operand?.let { left ->
                         // Trailing block-argument function call: the leading '{' is already consumed,
@@ -6042,12 +6082,14 @@ class Compiler(
             is ConstRef -> {
                 val range = ref.constValue as? ObjRange ?: return null
                 if (!range.isIntRange) return null
+                if (range.step != null && !range.step.isNull) return null
                 val start = range.start?.toLong() ?: return null
                 val end = range.end?.toLong() ?: return null
                 val endExclusive = if (range.isEndInclusive) end + 1 else end
                 return ConstIntRange(start, endExclusive)
             }
             is RangeRef -> {
+                if (ref.step != null) return null
                 val start = constIntValueOrNull(ref.left) ?: return null
                 val end = constIntValueOrNull(ref.right) ?: return null
                 val endExclusive = if (ref.isEndInclusive) end + 1 else end
diff --git a/lynglib/src/commonMain/kotlin/net/sergeych/lyng/Parser.kt b/lynglib/src/commonMain/kotlin/net/sergeych/lyng/Parser.kt
index e52772e..7171862 100644
--- a/lynglib/src/commonMain/kotlin/net/sergeych/lyng/Parser.kt
+++ b/lynglib/src/commonMain/kotlin/net/sergeych/lyng/Parser.kt
@@ -358,6 +358,7 @@ private class Parser(fromPos: Pos) {
                             "in" -> Token("in", from, Token.Type.IN)
                             "is" -> Token("is", from, Token.Type.IS)
                             "by" -> Token("by", from, Token.Type.BY)
+                            "step" -> Token("step", from, Token.Type.STEP)
                             "object" -> Token("object", from, Token.Type.OBJECT)
                             "as" -> {
                                 // support both `as` and tight `as?` without spaces
@@ -597,4 +598,4 @@ private class Parser(fromPos: Pos) {
             loadToEndOfLine()
     }
 
-}
\ No newline at end of file
+}
diff --git a/lynglib/src/commonMain/kotlin/net/sergeych/lyng/Token.kt b/lynglib/src/commonMain/kotlin/net/sergeych/lyng/Token.kt
index 19dcf32..74f7e7e 100644
--- a/lynglib/src/commonMain/kotlin/net/sergeych/lyng/Token.kt
+++ b/lynglib/src/commonMain/kotlin/net/sergeych/lyng/Token.kt
@@ -36,7 +36,7 @@ data class Token(val value: String, val pos: Pos, val type: Type) {
         PLUS, MINUS, STAR, SLASH, PERCENT,
         ASSIGN, PLUSASSIGN, MINUSASSIGN, STARASSIGN, SLASHASSIGN, PERCENTASSIGN, IFNULLASSIGN,
         PLUS2, MINUS2,
-        IN, NOTIN, IS, NOTIS, BY,
+        IN, NOTIN, IS, NOTIS, BY, STEP,
         EQ, NEQ, LT, LTE, GT, GTE, REF_EQ, REF_NEQ, MATCH, NOTMATCH,
         SHUTTLE,
         AND, BITAND, OR, BITOR, BITXOR, NOT, BITNOT, DOT, ARROW, EQARROW, QUESTION, COLONCOLON,
@@ -59,4 +59,4 @@ data class Token(val value: String, val pos: Pos, val type: Type) {
     companion object {
 //        fun eof(parser: Parser) = Token("", parser.currentPos, Type.EOF)
     }
-}
\ No newline at end of file
+}
diff --git a/lynglib/src/commonMain/kotlin/net/sergeych/lyng/bytecode/BytecodeCompiler.kt b/lynglib/src/commonMain/kotlin/net/sergeych/lyng/bytecode/BytecodeCompiler.kt
index 76b30b7..6c4c1dd 100644
--- a/lynglib/src/commonMain/kotlin/net/sergeych/lyng/bytecode/BytecodeCompiler.kt
+++ b/lynglib/src/commonMain/kotlin/net/sergeych/lyng/bytecode/BytecodeCompiler.kt
@@ -2202,8 +2202,17 @@ class BytecodeCompiler(
         val inclusiveSlot = allocSlot()
         val inclusiveId = builder.addConst(BytecodeConst.Bool(ref.isEndInclusive))
         builder.emit(Opcode.CONST_BOOL, inclusiveId, inclusiveSlot)
+        val stepSlot = if (ref.step != null) {
+            val step = compileRefWithFallback(ref.step, null, Pos.builtIn) ?: return null
+            ensureObjSlot(step).slot
+        } else {
+            val slot = allocSlot()
+            builder.emit(Opcode.CONST_NULL, slot)
+            updateSlotType(slot, SlotType.OBJ)
+            slot
+        }
         val dst = allocSlot()
-        builder.emit(Opcode.MAKE_RANGE, startSlot, endSlot, inclusiveSlot, dst)
+        builder.emit(Opcode.MAKE_RANGE, startSlot, endSlot, inclusiveSlot, stepSlot, dst)
         updateSlotType(dst, SlotType.OBJ)
         slotObjClass[dst] = ObjRange.type
         return CompiledValue(dst, SlotType.OBJ)
@@ -5508,7 +5517,8 @@ class BytecodeCompiler(
     private fun extractRangeRef(source: Statement): RangeRef? {
         val target = if (source is BytecodeStatement) source.original else source
         val expr = target as? ExpressionStatement ?: return null
-        return expr.ref as? RangeRef
+        val ref = expr.ref as? RangeRef ?: return null
+        return if (ref.step != null) null else ref
     }
 
     private fun extractDeclaredRange(stmt: Statement?): RangeRef? {
@@ -5519,6 +5529,7 @@ class BytecodeCompiler(
         if (ref is RangeRef) return ref
         if (ref is ConstRef) {
             val range = ref.constValue as? ObjRange ?: return null
+            if (range.step != null && !range.step.isNull) return null
             val start = range.start as? ObjInt ?: return null
             val end = range.end as? ObjInt ?: return null
             val left = ConstRef(start.asReadonly)
diff --git a/lynglib/src/commonMain/kotlin/net/sergeych/lyng/bytecode/CmdBuilder.kt b/lynglib/src/commonMain/kotlin/net/sergeych/lyng/bytecode/CmdBuilder.kt
index 59dbd59..6558a8e 100644
--- a/lynglib/src/commonMain/kotlin/net/sergeych/lyng/bytecode/CmdBuilder.kt
+++ b/lynglib/src/commonMain/kotlin/net/sergeych/lyng/bytecode/CmdBuilder.kt
@@ -183,7 +183,7 @@ class CmdBuilder {
             Opcode.SET_INDEX ->
                 listOf(OperandKind.SLOT, OperandKind.SLOT, OperandKind.SLOT)
             Opcode.MAKE_RANGE ->
-                listOf(OperandKind.SLOT, OperandKind.SLOT, OperandKind.SLOT, OperandKind.SLOT)
+                listOf(OperandKind.SLOT, OperandKind.SLOT, OperandKind.SLOT, OperandKind.SLOT, OperandKind.SLOT)
             Opcode.LIST_LITERAL ->
                 listOf(OperandKind.CONST, OperandKind.SLOT, OperandKind.COUNT, OperandKind.SLOT)
             Opcode.GET_MEMBER_SLOT ->
@@ -232,7 +232,7 @@ class CmdBuilder {
             Opcode.RANGE_INT_BOUNDS -> CmdRangeIntBounds(operands[0], operands[1], operands[2], operands[3])
             Opcode.LOAD_THIS -> CmdLoadThis(operands[0])
             Opcode.LOAD_THIS_VARIANT -> CmdLoadThisVariant(operands[0], operands[1])
-            Opcode.MAKE_RANGE -> CmdMakeRange(operands[0], operands[1], operands[2], operands[3])
+            Opcode.MAKE_RANGE -> CmdMakeRange(operands[0], operands[1], operands[2], operands[3], operands[4])
             Opcode.CHECK_IS -> CmdCheckIs(operands[0], operands[1], operands[2])
             Opcode.ASSERT_IS -> CmdAssertIs(operands[0], operands[1])
             Opcode.MAKE_QUALIFIED_VIEW -> CmdMakeQualifiedView(operands[0], operands[1], operands[2])
diff --git a/lynglib/src/commonMain/kotlin/net/sergeych/lyng/bytecode/CmdDisassembler.kt b/lynglib/src/commonMain/kotlin/net/sergeych/lyng/bytecode/CmdDisassembler.kt
index f126bf9..9b30177 100644
--- a/lynglib/src/commonMain/kotlin/net/sergeych/lyng/bytecode/CmdDisassembler.kt
+++ b/lynglib/src/commonMain/kotlin/net/sergeych/lyng/bytecode/CmdDisassembler.kt
@@ -76,7 +76,13 @@ object CmdDisassembler {
             is CmdAssertIs -> Opcode.ASSERT_IS to intArrayOf(cmd.objSlot, cmd.typeSlot)
             is CmdMakeQualifiedView -> Opcode.MAKE_QUALIFIED_VIEW to intArrayOf(cmd.objSlot, cmd.typeSlot, cmd.dst)
             is CmdRangeIntBounds -> Opcode.RANGE_INT_BOUNDS to intArrayOf(cmd.src, cmd.startSlot, cmd.endSlot, cmd.okSlot)
-            is CmdMakeRange -> Opcode.MAKE_RANGE to intArrayOf(cmd.startSlot, cmd.endSlot, cmd.inclusiveSlot, cmd.dst)
+            is CmdMakeRange -> Opcode.MAKE_RANGE to intArrayOf(
+                cmd.startSlot,
+                cmd.endSlot,
+                cmd.inclusiveSlot,
+                cmd.stepSlot,
+                cmd.dst
+            )
             is CmdResolveScopeSlot -> Opcode.RESOLVE_SCOPE_SLOT to intArrayOf(cmd.scopeSlot, cmd.addrSlot)
             is CmdLoadObjAddr -> Opcode.LOAD_OBJ_ADDR to intArrayOf(cmd.addrSlot, cmd.dst)
             is CmdStoreObjAddr -> Opcode.STORE_OBJ_ADDR to intArrayOf(cmd.src, cmd.addrSlot)
@@ -216,8 +222,10 @@ object CmdDisassembler {
                 listOf(OperandKind.SLOT, OperandKind.SLOT)
             Opcode.CHECK_IS, Opcode.MAKE_QUALIFIED_VIEW ->
                 listOf(OperandKind.SLOT, OperandKind.SLOT, OperandKind.SLOT)
-            Opcode.RANGE_INT_BOUNDS, Opcode.MAKE_RANGE ->
+            Opcode.RANGE_INT_BOUNDS ->
                 listOf(OperandKind.SLOT, OperandKind.SLOT, OperandKind.SLOT, OperandKind.SLOT)
+            Opcode.MAKE_RANGE ->
+                listOf(OperandKind.SLOT, OperandKind.SLOT, OperandKind.SLOT, OperandKind.SLOT, OperandKind.SLOT)
             Opcode.RET_LABEL, Opcode.THROW ->
                 listOf(OperandKind.CONST, OperandKind.SLOT)
             Opcode.RESOLVE_SCOPE_SLOT ->
diff --git a/lynglib/src/commonMain/kotlin/net/sergeych/lyng/bytecode/CmdRuntime.kt b/lynglib/src/commonMain/kotlin/net/sergeych/lyng/bytecode/CmdRuntime.kt
index 1db8b39..56c8d33 100644
--- a/lynglib/src/commonMain/kotlin/net/sergeych/lyng/bytecode/CmdRuntime.kt
+++ b/lynglib/src/commonMain/kotlin/net/sergeych/lyng/bytecode/CmdRuntime.kt
@@ -171,13 +171,16 @@ class CmdMakeRange(
     internal val startSlot: Int,
     internal val endSlot: Int,
     internal val inclusiveSlot: Int,
+    internal val stepSlot: Int,
     internal val dst: Int,
 ) : Cmd() {
     override suspend fun perform(frame: CmdFrame) {
         val start = frame.slotToObj(startSlot)
         val end = frame.slotToObj(endSlot)
         val inclusive = frame.slotToObj(inclusiveSlot).toBool()
-        frame.storeObjResult(dst, ObjRange(start, end, isEndInclusive = inclusive))
+        val stepObj = frame.slotToObj(stepSlot)
+        val step = if (stepObj.isNull) null else stepObj
+        frame.storeObjResult(dst, ObjRange(start, end, isEndInclusive = inclusive, step = step))
         return
     }
 }
diff --git a/lynglib/src/commonMain/kotlin/net/sergeych/lyng/highlight/SimpleLyngHighlighter.kt b/lynglib/src/commonMain/kotlin/net/sergeych/lyng/highlight/SimpleLyngHighlighter.kt
index b29938a..956e187 100644
--- a/lynglib/src/commonMain/kotlin/net/sergeych/lyng/highlight/SimpleLyngHighlighter.kt
+++ b/lynglib/src/commonMain/kotlin/net/sergeych/lyng/highlight/SimpleLyngHighlighter.kt
@@ -41,9 +41,9 @@ private val fallbackKeywordIds = setOf(
     // boolean operators
     "and", "or", "not",
     // declarations & modifiers
-    "fun", "fn", "class", "interface", "enum", "val", "var", "import", "package",
+    "fun", "fn", "class", "interface", "enum", "val", "var", "type", "import", "package",
     "abstract", "closed", "override", "public", "lazy", "dynamic",
-    "private", "protected", "static", "open", "extern", "init", "get", "set", "by",
+    "private", "protected", "static", "open", "extern", "init", "get", "set", "by", "step",
     // control flow and misc
     "if", "else", "when", "while", "do", "for", "try", "catch", "finally",
     "throw", "return", "break", "continue", "this", "null", "true", "false", "unset"
@@ -74,7 +74,7 @@ private fun kindOf(type: Type, value: String): HighlightKind? = when (type) {
     Type.COMMA, Type.SEMICOLON, Type.COLON -> HighlightKind.Punctuation
 
     // textual control keywords
-    Type.IN, Type.NOTIN, Type.IS, Type.NOTIS, Type.AS, Type.ASNULL, Type.BY, Type.OBJECT,
+    Type.IN, Type.NOTIN, Type.IS, Type.NOTIS, Type.AS, Type.ASNULL, Type.BY, Type.STEP, Type.OBJECT,
     Type.AND, Type.OR, Type.NOT -> HighlightKind.Keyword
 
     // labels / annotations
diff --git a/lynglib/src/commonMain/kotlin/net/sergeych/lyng/obj/ObjRange.kt b/lynglib/src/commonMain/kotlin/net/sergeych/lyng/obj/ObjRange.kt
index dc61c66..1ddb898 100644
--- a/lynglib/src/commonMain/kotlin/net/sergeych/lyng/obj/ObjRange.kt
+++ b/lynglib/src/commonMain/kotlin/net/sergeych/lyng/obj/ObjRange.kt
@@ -23,10 +23,16 @@ import net.sergeych.lyng.miniast.addFnDoc
 import net.sergeych.lyng.miniast.addPropertyDoc
 import net.sergeych.lyng.miniast.type
 
-class ObjRange(val start: Obj?, val end: Obj?, val isEndInclusive: Boolean) : Obj() {
+class ObjRange(
+    val start: Obj?,
+    val end: Obj?,
+    val isEndInclusive: Boolean,
+    val step: Obj? = null
+) : Obj() {
 
     val isOpenStart by lazy { start == null || start.isNull }
     val isOpenEnd by lazy { end == null || end.isNull }
+    val hasExplicitStep: Boolean get() = step != null && !step.isNull
 
     override val objClass: ObjClass get() = type
 
@@ -35,6 +41,9 @@ class ObjRange(val start: Obj?, val end: Obj?, val isEndInclusive: Boolean) : Ob
         result.append("${start?.inspect(scope) ?: '∞'} ..")
         if (!isEndInclusive) result.append('<')
         result.append(" ${end?.inspect(scope) ?: '∞'}")
+        if (hasExplicitStep) {
+            result.append(" step ${step?.inspect(scope)}")
+        }
         return ObjString(result.toString())
     }
 
@@ -64,26 +73,26 @@ class ObjRange(val start: Obj?, val end: Obj?, val isEndInclusive: Boolean) : Ob
         }
 
     suspend fun containsRange(scope: Scope, other: ObjRange): Boolean {
-        if (start != null) {
+        if (!isOpenStart) {
             // our start is not -∞ so other start should be GTE or is not contained:
-            if (other.start != null && start.compareTo(scope, other.start) > 0) return false
+            if (!other.isOpenStart && start!!.compareTo(scope, other.start!!) > 0) return false
         }
-        if (end != null) {
+        if (!isOpenEnd) {
             // same with the end: if it is open, it can't be contained in ours:
-            if (other.end == null) return false
+            if (other.isOpenEnd) return false
             // both exists, now there could be 4 cases:
             return when {
                 other.isEndInclusive && isEndInclusive ->
-                    end.compareTo(scope, other.end) >= 0
+                    end!!.compareTo(scope, other.end!!) >= 0
 
                 !other.isEndInclusive && !isEndInclusive ->
-                    end.compareTo(scope, other.end) >= 0
+                    end!!.compareTo(scope, other.end!!) >= 0
 
                 other.isEndInclusive && !isEndInclusive ->
-                    end.compareTo(scope, other.end) > 0
+                    end!!.compareTo(scope, other.end!!) > 0
 
                 !other.isEndInclusive && isEndInclusive ->
-                    end.compareTo(scope, other.end) >= 0
+                    end!!.compareTo(scope, other.end!!) >= 0
 
                 else -> throw IllegalStateException("unknown comparison")
             }
@@ -120,12 +129,12 @@ class ObjRange(val start: Obj?, val end: Obj?, val isEndInclusive: Boolean) : Ob
             }
         }
 
-        if (start == null && end == null) return true
-        if (start != null) {
-            if (start.compareTo(scope, other) > 0) return false
+        if (isOpenStart && isOpenEnd) return true
+        if (!isOpenStart) {
+            if (start!!.compareTo(scope, other) > 0) return false
         }
-        if (end != null) {
-            val cmp = end.compareTo(scope, other)
+        if (!isOpenEnd) {
+            val cmp = end!!.compareTo(scope, other)
             if (isEndInclusive && cmp < 0 || !isEndInclusive && cmp <= 0) return false
         }
         return true
@@ -140,7 +149,7 @@ class ObjRange(val start: Obj?, val end: Obj?, val isEndInclusive: Boolean) : Ob
     }
 
     override suspend fun enumerate(scope: Scope, callback: suspend (Obj) -> Boolean) {
-        if (start is ObjInt && end is ObjInt) {
+        if (!hasExplicitStep && start is ObjInt && end is ObjInt) {
             val s = start.value
             val e = end.value
             if (isEndInclusive) {
@@ -152,7 +161,7 @@ class ObjRange(val start: Obj?, val end: Obj?, val isEndInclusive: Boolean) : Ob
                     if (!callback(ObjInt.of(i))) break
                 }
             }
-        } else if (start is ObjChar && end is ObjChar) {
+        } else if (!hasExplicitStep && start is ObjChar && end is ObjChar) {
             val s = start.value
             val e = end.value
             if (isEndInclusive) {
@@ -171,7 +180,11 @@ class ObjRange(val start: Obj?, val end: Obj?, val isEndInclusive: Boolean) : Ob
 
     override suspend fun compareTo(scope: Scope, other: Obj): Int {
         return (other as? ObjRange)?.let {
-            if( start == other.start && end == other.end ) 0 else -1
+            if (start == other.start &&
+                end == other.end &&
+                isEndInclusive == other.isEndInclusive &&
+                step == other.step
+            ) 0 else -1
         }
             ?: -1
     }
@@ -180,6 +193,7 @@ class ObjRange(val start: Obj?, val end: Obj?, val isEndInclusive: Boolean) : Ob
         var result = start?.hashCode() ?: 0
         result = 31 * result + (end?.hashCode() ?: 0)
         result = 31 * result + isEndInclusive.hashCode()
+        result = 31 * result + (step?.hashCode() ?: 0)
         return result
     }
 
@@ -192,6 +206,7 @@ class ObjRange(val start: Obj?, val end: Obj?, val isEndInclusive: Boolean) : Ob
         if (start != other.start) return false
         if (end != other.end) return false
         if (isEndInclusive != other.isEndInclusive) return false
+        if (step != other.step) return false
 
         return true
     }
@@ -213,12 +228,19 @@ class ObjRange(val start: Obj?, val end: Obj?, val isEndInclusive: Boolean) : Ob
                 moduleName = "lyng.stdlib",
                 getter = { thisAs<ObjRange>().end ?: ObjNull }
             )
+            addPropertyDoc(
+                name = "step",
+                doc = "Explicit step for iteration, or null if implicit.",
+                type = type("lyng.Any", nullable = true),
+                moduleName = "lyng.stdlib",
+                getter = { thisAs<ObjRange>().step ?: ObjNull }
+            )
             addPropertyDoc(
                 name = "isOpen",
                 doc = "Whether the range is open on either side (no start or no end).",
                 type = type("lyng.Bool"),
                 moduleName = "lyng.stdlib",
-                getter = { thisAs<ObjRange>().let { it.start == null || it.end == null }.toObj() }
+                getter = { thisAs<ObjRange>().let { it.isOpenStart || it.isOpenEnd }.toObj() }
             )
             addPropertyDoc(
                 name = "isIntRange",
@@ -248,20 +270,78 @@ class ObjRange(val start: Obj?, val end: Obj?, val isEndInclusive: Boolean) : Ob
                 moduleName = "lyng.stdlib"
             ) {
                 val self = thisAs<ObjRange>()
-                if (net.sergeych.lyng.PerfFlags.RANGE_FAST_ITER) {
-                    val s = self.start
-                    val e = self.end
-                    if (s is ObjInt && e is ObjInt) {
-                        val start = s.value.toInt()
-                        val endExclusive = (if (self.isEndInclusive) e.value.toInt() + 1 else e.value.toInt())
-                        // Only for ascending simple ranges; fall back otherwise
-                        if (start <= endExclusive) {
-                            return@addFnDoc ObjFastIntRangeIterator(start, endExclusive)
-                        }
-                    }
-                }
-                ObjRangeIterator(self).apply { init() }
+                self.buildIterator(this)
             }
         }
     }
+
+    private fun explicitStepOrNull(): Obj? = step?.takeUnless { it.isNull }
+
+    private suspend fun resolveStep(scope: Scope, explicitStep: Obj?): Obj {
+        val startObj = start ?: ObjNull
+        if (explicitStep != null) {
+            if (explicitStep is Numeric && explicitStep.doubleValue == 0.0) {
+                scope.raiseIllegalState("Range step cannot be zero")
+            }
+            if (startObj is ObjChar && explicitStep !is ObjInt) {
+                scope.raiseIllegalState("Char range step must be Int")
+            }
+            if (startObj is Numeric && explicitStep !is Numeric) {
+                scope.raiseIllegalState("Numeric range step must be numeric")
+            }
+            return explicitStep
+        }
+        if (startObj is ObjInt) {
+            val cmp = if (end == null || end.isNull) 0 else startObj.compareTo(scope, end)
+            val dir = if (cmp >= 0) -1 else 1
+            return ObjInt.of(dir.toLong())
+        }
+        if (startObj is ObjChar) {
+            val endChar = end as? ObjChar
+                ?: scope.raiseIllegalState("Char range requires Char end to infer step")
+            val dir = if (startObj.value >= endChar.value) -1 else 1
+            return ObjInt.of(dir.toLong())
+        }
+        if (startObj is ObjReal) {
+            scope.raiseIllegalState("Real range requires explicit step")
+        }
+        scope.raiseIllegalState("Range of ${startObj.objClass.className} requires explicit step")
+    }
+
+    private suspend fun directionMismatch(scope: Scope, step: Obj): Boolean {
+        if (end == null || end.isNull) return false
+        val startObj = start ?: ObjNull
+        if (startObj is ObjChar && end !is ObjChar) return false
+        val cmp = startObj.compareTo(scope, end)
+        if (cmp == -2) return false
+        if (cmp == 0) return false
+        val stepSign = when {
+            startObj is ObjChar && step is ObjInt -> step.value.compareTo(0)
+            step is Numeric -> step.doubleValue.compareTo(0.0)
+            else -> return false
+        }
+        return (cmp < 0 && stepSign < 0) || (cmp > 0 && stepSign > 0)
+    }
+
+    suspend fun buildIterator(scope: Scope): Obj {
+        if (isOpenStart) scope.raiseIllegalState("Range with open start is not iterable")
+        val explicitStep = explicitStepOrNull()
+        if (isOpenEnd && explicitStep == null) {
+            scope.raiseIllegalState("Open-ended range requires explicit step to iterate")
+        }
+        val stepValue = resolveStep(scope, explicitStep)
+        val mismatch = directionMismatch(scope, stepValue)
+        if (net.sergeych.lyng.PerfFlags.RANGE_FAST_ITER) {
+            val s = start
+            val e = end
+            if (!mismatch && stepValue is ObjInt && stepValue.value == 1L && s is ObjInt && e is ObjInt) {
+                val startVal = s.value.toInt()
+                val endExclusive = (if (isEndInclusive) e.value.toInt() + 1 else e.value.toInt())
+                if (startVal <= endExclusive) {
+                    return ObjFastIntRangeIterator(startVal, endExclusive)
+                }
+            }
+        }
+        return ObjRangeIterator(this, stepValue, mismatch)
+    }
 }
diff --git a/lynglib/src/commonMain/kotlin/net/sergeych/lyng/obj/ObjRangeIterator.kt b/lynglib/src/commonMain/kotlin/net/sergeych/lyng/obj/ObjRangeIterator.kt
index 5a38285..d4ef07a 100644
--- a/lynglib/src/commonMain/kotlin/net/sergeych/lyng/obj/ObjRangeIterator.kt
+++ b/lynglib/src/commonMain/kotlin/net/sergeych/lyng/obj/ObjRangeIterator.kt
@@ -17,57 +17,55 @@
 
 package net.sergeych.lyng.obj
 
-import net.sergeych.lyng.PerfFlags
 import net.sergeych.lyng.Scope
 
-class ObjRangeIterator(val self: ObjRange) : Obj() {
+class ObjRangeIterator(
+    private val self: ObjRange,
+    private val step: Obj,
+    private val directionMismatch: Boolean
+) : Obj() {
 
-    private var nextIndex = 0
-    private var lastIndex = 0
-    private var isCharRange: Boolean = false
+    private var current: Obj? = null
+    private var initialized = false
 
     override val objClass: ObjClass get() = type
 
-    fun Scope.init() {
-        val s = self.start
-        val e = self.end
-        if (s is ObjInt && e is ObjInt) {
-            lastIndex = if (self.isEndInclusive)
-                (e.value - s.value + 1).toInt()
-            else
-                (e.value - s.value).toInt()
-        } else if (s is ObjChar && e is ObjChar) {
-            isCharRange = true
-            lastIndex = if (self.isEndInclusive)
-                (e.value.code - s.value.code + 1)
-            else
-                (e.value.code - s.value.code)
-        } else {
-            raiseError("not implemented iterator for range of $this")
+    private fun ensureInit() {
+        if (!initialized) {
+            current = self.start ?: ObjNull
+            initialized = true
         }
     }
 
-    fun hasNext(): Boolean = nextIndex < lastIndex
+    suspend fun hasNext(scope: Scope): Boolean {
+        if (directionMismatch) return false
+        ensureInit()
+        val cur = current ?: return false
+        return self.contains(scope, cur)
+    }
 
-    fun next(scope: Scope): Obj =
-        if (nextIndex < lastIndex) {
-            val start = self.start
-            val x = if (start is ObjInt)
-                start.value + nextIndex++
-            else if (start is ObjChar)
-                start.value.code.toLong() + nextIndex++
-            else
-                scope.raiseError("iterator error: unsupported range start")
-            if (isCharRange) ObjChar(x.toInt().toChar()) else ObjInt.of(x)
-        }
-        else {
+    suspend fun next(scope: Scope): Obj {
+        if (!hasNext(scope)) {
             scope.raiseError(ObjIterationFinishedException(scope))
         }
+        val result = current ?: scope.raiseError("iterator error: missing current value")
+        current = advance(scope, result)
+        return result
+    }
+
+    private suspend fun advance(scope: Scope, value: Obj): Obj =
+        if (value is ObjChar) {
+            val delta = (step as? ObjInt)
+                ?: scope.raiseIllegalState("Char range step must be Int")
+            ObjChar((value.value.code + delta.value.toInt()).toChar())
+        } else {
+            value.plus(scope, step)
+        }
 
     companion object {
         val type = ObjClass("RangeIterator", ObjIterator).apply {
             addFn("hasNext") {
-                thisAs<ObjRangeIterator>().hasNext().toObj()
+                thisAs<ObjRangeIterator>().hasNext(this).toObj()
             }
             addFn("next") {
                 thisAs<ObjRangeIterator>().next(this)
diff --git a/lynglib/src/commonMain/kotlin/net/sergeych/lyng/obj/ObjRef.kt b/lynglib/src/commonMain/kotlin/net/sergeych/lyng/obj/ObjRef.kt
index dd4e0e0..8c7bb50 100644
--- a/lynglib/src/commonMain/kotlin/net/sergeych/lyng/obj/ObjRef.kt
+++ b/lynglib/src/commonMain/kotlin/net/sergeych/lyng/obj/ObjRef.kt
@@ -2495,7 +2495,8 @@ class MapLiteralRef(private val entries: List<MapLiteralEntry>) : ObjRef {
 class RangeRef(
     internal val left: ObjRef?,
     internal val right: ObjRef?,
-    internal val isEndInclusive: Boolean
+    internal val isEndInclusive: Boolean,
+    internal val step: ObjRef? = null
 ) : ObjRef {
     override suspend fun get(scope: Scope): ObjRecord {
         return evalValue(scope).asReadonly
@@ -2504,7 +2505,8 @@ class RangeRef(
     override suspend fun evalValue(scope: Scope): Obj {
         val l = left?.evalValue(scope) ?: ObjNull
         val r = right?.evalValue(scope) ?: ObjNull
-        return ObjRange(l, r, isEndInclusive = isEndInclusive)
+        val st = step?.evalValue(scope)
+        return ObjRange(l, r, isEndInclusive = isEndInclusive, step = st)
     }
 }
 
diff --git a/lynglib/src/commonMain/kotlin/net/sergeych/lyng/statements.kt b/lynglib/src/commonMain/kotlin/net/sergeych/lyng/statements.kt
index 782ea9a..92e9308 100644
--- a/lynglib/src/commonMain/kotlin/net/sergeych/lyng/statements.kt
+++ b/lynglib/src/commonMain/kotlin/net/sergeych/lyng/statements.kt
@@ -125,7 +125,7 @@ class ForInStatement(
         }
 
         val sourceObj = source.execute(forContext)
-        return if (sourceObj is ObjRange && sourceObj.isIntRange && PerfFlags.PRIMITIVE_FASTOPS) {
+        return if (sourceObj is ObjRange && sourceObj.isIntRange && !sourceObj.hasExplicitStep && PerfFlags.PRIMITIVE_FASTOPS) {
             loopIntRange(
                 forContext,
                 sourceObj.start!!.toLong(),
diff --git a/lynglib/src/commonTest/kotlin/ScriptTest.kt b/lynglib/src/commonTest/kotlin/ScriptTest.kt
index 69759e7..1f0bcba 100644
--- a/lynglib/src/commonTest/kotlin/ScriptTest.kt
+++ b/lynglib/src/commonTest/kotlin/ScriptTest.kt
@@ -3457,6 +3457,26 @@ class ScriptTest {
         println(y.list)
     }
 
+    @Test
+    fun testRangeStepIteration() = runTest {
+        val ints = eval("""(1..5 step 2).toList()""") as ObjList
+        assertEquals(listOf(1, 3, 5), ints.list.map { it.toInt() })
+        val chars = eval("""('a'..'e' step 2).toList()""") as ObjList
+        assertEquals(listOf('a', 'c', 'e'), chars.list.map { it.toString().single() })
+        val reals = eval("""(0.0..1.0 step 0.25).toList()""") as ObjList
+        assertEquals(listOf(0.0, 0.25, 0.5, 0.75, 1.0), reals.list.map { it.toDouble() })
+        val empty = eval("""(5..1 step 1).toList()""") as ObjList
+        assertEquals(0, empty.list.size)
+        val openEnd = eval("""(0.. step 1).take(3).toList()""") as ObjList
+        assertEquals(listOf(0, 1, 2), openEnd.list.map { it.toInt() })
+        assertFailsWith<ExecutionError> {
+            eval("""(0.0..1.0).toList()""")
+        }
+        assertFailsWith<ExecutionError> {
+            eval("""(0..).toList()""")
+        }
+    }
+
     @Test
     fun testMultilineStrings() = runTest {
         assertEquals(
diff --git a/lyngweb/src/jsMain/kotlin/net/sergeych/lyngweb/Highlight.kt b/lyngweb/src/jsMain/kotlin/net/sergeych/lyngweb/Highlight.kt
index bfff5aa..07adca7 100644
--- a/lyngweb/src/jsMain/kotlin/net/sergeych/lyngweb/Highlight.kt
+++ b/lyngweb/src/jsMain/kotlin/net/sergeych/lyngweb/Highlight.kt
@@ -21,14 +21,11 @@
 package net.sergeych.lyngweb
 
 import kotlinx.browser.window
-import net.sergeych.lyng.Compiler
-import net.sergeych.lyng.Source
-import net.sergeych.lyng.binding.Binder
-import net.sergeych.lyng.binding.SymbolKind
 import net.sergeych.lyng.highlight.HighlightKind
 import net.sergeych.lyng.highlight.SimpleLyngHighlighter
 import net.sergeych.lyng.highlight.offsetOf
-import net.sergeych.lyng.miniast.*
+import net.sergeych.lyng.tools.LyngLanguageTools
+import net.sergeych.lyng.tools.LyngSemanticKind
 import org.w3c.dom.HTMLStyleElement
 
 fun ensureBootstrapCodeBlocks(html: String): String {
@@ -339,164 +336,34 @@ fun applyLyngHighlightToText(text: String): String {
  */
 suspend fun applyLyngHighlightToTextAst(text: String): String {
     return try {
-        // Ensure CSS present
         ensureLyngHighlightStyles()
-        val source = Source("<web>", text)
-        // Token baseline
-        val tokenSpans = SimpleLyngHighlighter().highlight(text)
+        val analysis = LyngLanguageTools.analyze(text, "<web>")
+        val source = analysis.source
+        val tokenSpans = analysis.lexicalHighlights
         if (tokenSpans.isEmpty()) return htmlEscape(text)
 
-        // Build Mini-AST
-        val sink = MiniAstBuilder()
-        Compiler.compileWithMini(text, sink)
-        val mini = sink.build()
-
-        // Collect overrides from AST and Binding with precise offsets
         val overrides = HashMap<Pair<Int, Int>, String>()
-        fun putName(startPos: net.sergeych.lyng.Pos, name: String, cls: String) {
-            val s = source.offsetOf(startPos)
-            val e = s + name.length
-            if (s >= 0 && e <= text.length && s < e) overrides[s to e] = cls
+        fun classForSemantic(kind: LyngSemanticKind): String? = when (kind) {
+            LyngSemanticKind.Function -> "hl-fn"
+            LyngSemanticKind.Class, LyngSemanticKind.Enum, LyngSemanticKind.TypeAlias -> "hl-class"
+            LyngSemanticKind.Value -> "hl-val"
+            LyngSemanticKind.Variable -> "hl-var"
+            LyngSemanticKind.Parameter -> "hl-param"
+            LyngSemanticKind.TypeRef -> "hl-ty"
+            LyngSemanticKind.EnumConstant -> "hl-enumc"
         }
-        // Declarations
-        mini?.declarations?.forEach { d ->
-            when (d) {
-                is MiniFunDecl -> putName(d.nameStart, d.name, "hl-fn")
-                is MiniClassDecl -> putName(d.nameStart, d.name, "hl-class")
-                is net.sergeych.lyng.miniast.MiniValDecl -> putName(d.nameStart, d.name, if (d.mutable) "hl-var" else "hl-val")
-                is MiniEnumDecl -> putName(d.nameStart, d.name, "hl-class")
-            }
+
+        LyngLanguageTools.semanticHighlights(analysis).forEach { s ->
+            classForSemantic(s.kind)?.let { overrides[s.range.start to s.range.endExclusive] = it }
         }
-        // Imports: color each segment as directive/path
-        mini?.imports?.forEach { imp ->
+
+        analysis.mini?.imports?.forEach { imp ->
             imp.segments.forEach { seg ->
                 val s = source.offsetOf(seg.range.start)
                 val e = source.offsetOf(seg.range.end)
                 if (s >= 0 && e <= text.length && s < e) overrides[s to e] = "hl-dir"
             }
         }
-        // Parameters
-        mini?.declarations?.filterIsInstance<MiniFunDecl>()?.forEach { fn ->
-            fn.params.forEach { p -> putName(p.nameStart, p.name, "hl-param") }
-        }
-        // Type name segments
-        fun addTypeSegments(t: net.sergeych.lyng.miniast.MiniTypeRef?) {
-            when (t) {
-                is MiniTypeName -> t.segments.forEach { seg ->
-                    val s = source.offsetOf(seg.range.start)
-                    val e = s + seg.name.length
-                    if (s >= 0 && e <= text.length && s < e) overrides[s to e] = "hl-ty"
-                }
-                is net.sergeych.lyng.miniast.MiniGenericType -> {
-                    addTypeSegments(t.base)
-                    t.args.forEach { addTypeSegments(it) }
-                }
-                else -> {}
-            }
-        }
-        mini?.declarations?.forEach { d ->
-            when (d) {
-                is MiniFunDecl -> {
-                    addTypeSegments(d.returnType)
-                    d.params.forEach { addTypeSegments(it.type) }
-                }
-                is net.sergeych.lyng.miniast.MiniValDecl -> addTypeSegments(d.type)
-                is MiniClassDecl -> {}
-                is MiniEnumDecl -> {}
-            }
-        }
-
-        // Apply binder results to mark usages by semantic kind (params, locals, top-level, functions, classes)
-        try {
-            if (mini != null) {
-                val binding = Binder.bind(text, mini)
-                val symbolsById = binding.symbols.associateBy { it.id }
-                // Map decl ranges to avoid overriding declarations
-                val declKeys = HashSet<Pair<Int, Int>>()
-                for (sym in binding.symbols) {
-                    declKeys += (sym.declStart to sym.declEnd)
-                }
-                fun classForKind(k: SymbolKind): String? = when (k) {
-                    SymbolKind.Function -> "hl-fn"
-                    SymbolKind.Class, SymbolKind.Enum -> "hl-class"
-                    SymbolKind.Parameter -> "hl-param"
-                    SymbolKind.Value -> "hl-val"
-                    SymbolKind.Variable -> "hl-var"
-                }
-                for (ref in binding.references) {
-                    val key = ref.start to ref.end
-                    if (declKeys.contains(key)) continue
-                    if (!overrides.containsKey(key)) {
-                        val sym = symbolsById[ref.symbolId]
-                        val cls = sym?.let { classForKind(it.kind) }
-                        if (cls != null) overrides[key] = cls
-                    }
-                }
-            }
-        } catch (_: Throwable) {
-            // Binder is best-effort; ignore on any failure
-        }
-
-        fun isFollowedByParen(rangeEnd: Int): Boolean {
-            var i = rangeEnd
-            while (i < text.length) {
-                val ch = text[i]
-                if (ch == ' ' || ch == '\t' || ch == '\r' || ch == '\n') { i++; continue }
-                return ch == '('
-            }
-            return false
-        }
-
-        fun isFollowedByBlock(rangeEnd: Int): Boolean {
-            var i = rangeEnd
-            while (i < text.length) {
-                val ch = text[i]
-                if (ch == ' ' || ch == '\t' || ch == '\r' || ch == '\n') { i++; continue }
-                return ch == '{'
-            }
-            return false
-        }
-
-        // First: mark function call-sites (identifier immediately followed by '('), best-effort.
-        // Do this before vars/params so it takes precedence where both could match.
-        run {
-            for (s in tokenSpans) {
-                if (s.kind == HighlightKind.Identifier) {
-                    val key = s.range.start to s.range.endExclusive
-                    if (!overrides.containsKey(key)) {
-                        if (isFollowedByParen(s.range.endExclusive) || isFollowedByBlock(s.range.endExclusive)) {
-                            overrides[key] = "hl-fn"
-                        }
-                    }
-                }
-            }
-        }
-
-        // Highlight usages of top-level vals/vars and parameters (best-effort, no binder yet)
-        val nameRoleMap = HashMap<String, String>(8)
-        mini?.declarations?.forEach { d ->
-            when (d) {
-                is net.sergeych.lyng.miniast.MiniValDecl -> nameRoleMap[d.name] = if (d.mutable) "hl-var" else "hl-val"
-                is MiniFunDecl -> d.params.forEach { p -> nameRoleMap[p.name] = "hl-param" }
-                else -> {}
-            }
-        }
-        // For every identifier token not already overridden, apply role based on known names
-        for (s in tokenSpans) {
-            if (s.kind == HighlightKind.Identifier) {
-                val key = s.range.start to s.range.endExclusive
-                if (!overrides.containsKey(key)) {
-                    val ident = text.substring(s.range.start, s.range.endExclusive)
-                    val cls = nameRoleMap[ident]
-                    if (cls != null) {
-                        // Avoid marking function call sites as vars/params
-                        if (!isFollowedByParen(s.range.endExclusive)) {
-                            overrides[key] = cls
-                        }
-                    }
-                }
-            }
-        }
 
         // Render merging overrides
         val sb = StringBuilder(text.length + tokenSpans.size * 16)
@@ -534,7 +401,7 @@ private fun detectDeclarationAndParamOverrides(text: String): Map<Pair<Int, Int>
         "if", "else", "while", "do", "for", "when", "try", "catch", "finally",
         "throw", "return", "break", "continue", "in", "is", "as", "as?", "not",
         "true", "false", "null", "private", "protected", "abstract", "closed", "override", "open", "extern", "static",
-        "init", "get", "set", "Unset", "by"
+        "init", "get", "set", "Unset", "by", "step"
     )
     fun skipWs(idx0: Int): Int {
         var idx = idx0