Add receiver-stack function types

docs/OOP.md

@@ -454,6 +454,31 @@ Key rules and features:
   - For arbitrary receivers, use casts: `(expr as Type).member(...)` or `(expr as? Type)?.member(...)`.
   - Qualified access does not relax visibility.
 
+### Receiver-stack lambdas
+
+Qualified `this@Type` is also used outside inheritance when a lambda has multiple visible receivers.
+This is common in DSL-style builders.
+
+- `A & B` means one receiver value that implements both types.
+- `context(A, B) C.()->R` means a receiver stack:
+  - primary `this` is `C`
+  - outer/context receivers are `A`, then `B`
+- Unqualified lookup checks the primary receiver first.
+- If the primary receiver does not define a member and several outer/context receivers do, Lyng reports a compile-time ambiguity. Use `this@Type` to select one explicitly.
+
+Example:
+
+```lyng
+class Html { fun title() = "html" }
+class Head { fun title() = "head" }
+class Body
+
+val block: context(Html, Head) Body.()->String = {
+    // title()         // compile-time ambiguity: Html vs Head
+    this@Html.title()
+}
+```
+
 - Field inheritance (`val`/`var`) and collisions
   - Instance storage is kept per declaring class, internally disambiguated; unqualified read/write resolves to the first match in the resolution order (leftmost base).
   - Qualified read/write (via `this@Type` or casts) targets the chosen ancestor’s storage.

docs/ai_language_reference.md

@@ -164,7 +164,12 @@ Primary sources used: `lynglib/src/commonMain/kotlin/net/sergeych/lyng/{Parser,T
   - unions `A | B`
   - intersections `A & B`
   - function types `(A, B)->R` and receiver form `Receiver.(A)->R`
+  - receiver-stack function types via `context(A, B) Receiver.(P)->R`
   - variadics in function type via ellipsis (`T...`)
+- `A & B` means one value implementing both types.
+- `context(A, B) Receiver.(P)->R` is different: it declares an ordered implicit-receiver stack where `Receiver` is primary `this`, then `A`, then `B`.
+- Nested receiver lambdas keep outer receivers in scope; unqualified lookup prefers the innermost receiver, and `this@Type` can select an outer/context receiver explicitly.
+- If the primary receiver does not provide a member and multiple outer/context receivers do, the lookup is a compile-time ambiguity and must be disambiguated with `this@Type`.
 - Generics:
   - type params on classes/functions/type aliases
   - bounds via `:` with union/intersection expressions
@@ -217,6 +222,7 @@ Primary sources used: `lynglib/src/commonMain/kotlin/net/sergeych/lyng/{Parser,T
 - Disambiguation helpers are supported:
   - qualified this: `this@Base.member()`
   - cast view: `(obj as Base).member()`
+- In nested receiver lambdas, `this@Type` can target any receiver visible through the receiver stack, not just inheritance ancestors.
 - On unknown receiver types, compiler allows only Object-safe members:
   - `toString`, `toInspectString`, `let`, `also`, `apply`, `run`
 - Other members require known receiver type or explicit cast.

docs/tutorial.md

@@ -352,6 +352,40 @@ Sets `this` to the first argument and executes the block. Returns the value retu
     assertEquals(3, sum)
     >>> void
 
+Receiver lambdas can also keep outer receivers in scope. The primary receiver wins for unqualified lookup, and `this@Type`
+selects an outer receiver explicitly:
+
+    class Html { fun lang() = "en" }
+    class Body { fun lang() = "body" }
+
+    fun html(block: Html.()->String) = with(Html()) { block(this) }
+    fun body(block: Body.()->String) = with(Body()) { block(this) }
+
+    val result = html {
+        body {
+            lang() + ":" + this@Html.lang()
+        }
+    }
+    assertEquals("body:en", result)
+    >>> void
+
+You can declare the same requirement in a function type:
+
+    val block: context(Html) Body.()->String = {
+        lang() + ":" + this@Html.lang()
+    }
+
+If the primary receiver does not define a member and multiple outer/context receivers do, Lyng reports an ambiguity instead of picking one silently:
+
+    class A { fun title() = "a" }
+    class B { fun title() = "b" }
+    class C
+
+    val block: context(A, B) C.()->String = {
+        // title()  // compile-time ambiguity
+        this@A.title()
+    }
+
 ## run
 
 Executes a block after it returning the value passed by the block. for example, can be used with elvis operator: