diff --git a/src/commonMain/kotlin/net.sergeych.bintools/DataSink.kt b/src/commonMain/kotlin/net.sergeych.bintools/DataSink.kt
index 0ed3dea..4816af2 100644
--- a/src/commonMain/kotlin/net.sergeych.bintools/DataSink.kt
+++ b/src/commonMain/kotlin/net.sergeych.bintools/DataSink.kt
@@ -22,6 +22,11 @@ interface DataSink {
     fun writeBytes(data: ByteArray) {
         for(d in data) writeByte(d)
     }
+
+    fun writeVarUInt(value: UInt) { Varint.encodeUnsigned(value.toULong())}
+    fun writeVarInt(value: UInt) { Varint.encodeSigned(value.toLong())}
+    fun writeSmartUInt(value: UInt) { Smartint.encodeUnsigned(value.toULong())}
+    fun writeSmartInt(value: UInt) { Smartint.encodeSigned(value.toLong())}
 }
 
 inline fun <reified T:Any>DataSink.writeNumber(value: T) {
diff --git a/src/commonMain/kotlin/net.sergeych.bintools/DataSource.kt b/src/commonMain/kotlin/net.sergeych.bintools/DataSource.kt
index af9af64..408accc 100644
--- a/src/commonMain/kotlin/net.sergeych.bintools/DataSource.kt
+++ b/src/commonMain/kotlin/net.sergeych.bintools/DataSource.kt
@@ -34,6 +34,14 @@ interface DataSource {
 
     fun readFloat() = Float.fromBits(readI32()).toFloat()
 
+    fun readSmartUInt(): UInt = Smartint.decodeUnsigned(this).toUInt()
+    fun readSmartInt(): Int = Smartint.decodeSigned(this).toInt()
+
+    fun readVarUInt(): UInt = Varint.decodeUnsigned(this).toUInt()
+    fun readVarInt(): Int = Varint.decodeSigned(this).toInt()
+
+
+
 }
 
 fun ByteArray.toDataSource(): DataSource =
diff --git a/src/commonMain/kotlin/net.sergeych.bipack/BipackDecoder.kt b/src/commonMain/kotlin/net.sergeych.bipack/BipackDecoder.kt
index b11ffff..fddc8db 100644
--- a/src/commonMain/kotlin/net.sergeych.bipack/BipackDecoder.kt
+++ b/src/commonMain/kotlin/net.sergeych.bipack/BipackDecoder.kt
@@ -59,8 +59,8 @@ class BipackDecoder(val input: DataSource, var elementsCount: Int = 0,val isColl
         // CRC-calculating one, and the fields below are CRC protected too:
         var count = descriptor.elementsCount
         for (a in descriptor.annotations) {
-            if (a is ExtendableFormat)
-                count = source.readNumber<UInt>().toInt()
+            if (a is Extendable)
+                count = source.readSmartUInt().toInt()
             else if (a is Framed) {
                 val code = CRC.crc32(descriptor.serialName.encodeToByteArray())
                 // if we fail to read CRC, it is IO error, so DataSource.EndOfData will be
diff --git a/src/commonMain/kotlin/net.sergeych.bipack/BipackEncoder.kt b/src/commonMain/kotlin/net.sergeych.bipack/BipackEncoder.kt
index 158c361..4fd33b2 100644
--- a/src/commonMain/kotlin/net.sergeych.bipack/BipackEncoder.kt
+++ b/src/commonMain/kotlin/net.sergeych.bipack/BipackEncoder.kt
@@ -72,8 +72,8 @@ class BipackEncoder(val output: DataSink) : AbstractEncoder() {
                 sink.writeU32(
                     CRC.crc32(descriptor.serialName.encodeToByteArray())
                 )
-            } else if (a is ExtendableFormat) {
-                sink.writeNumber(descriptor.elementsCount.toUInt())
+            } else if (a is Extendable) {
+                sink.writeSmartUInt(descriptor.elementsCount.toUInt())
             }
         }
         return BipackEncoder(sink)
diff --git a/src/commonMain/kotlin/net.sergeych.bipack/annotations.kt b/src/commonMain/kotlin/net.sergeych.bipack/annotations.kt
index aa872be..4fe7ed1 100644
--- a/src/commonMain/kotlin/net.sergeych.bipack/annotations.kt
+++ b/src/commonMain/kotlin/net.sergeych.bipack/annotations.kt
@@ -15,7 +15,7 @@ import kotlinx.serialization.SerialInfo
  */
 @Target(AnnotationTarget.CLASS)
 @SerialInfo
-annotation class ExtendableFormat
+annotation class Extendable
 
 
 /**
@@ -29,7 +29,7 @@ annotation class Framed
 /**
  * Allow to CRC-protect structures (we suppose to use it with classes only). After the
  * data block its CRC32 will be written and checked. It is memory-wise: it calculates CRC
- * on the fly without buffering the data. If used with [Framed] and [ExtendableFormat] the extra
+ * on the fly without buffering the data. If used with [Framed] and [Extendable] the extra
  * data is protected too.
  *
  * __Common pitfalls__. When unpacking corrupted data protected this way, the not only [InvalidFrameCRCException]
diff --git a/src/commonTest/kotlin/bipack/BipackEncoderTest.kt b/src/commonTest/kotlin/bipack/BipackEncoderTest.kt
index ed747f2..0ac2da3 100644
--- a/src/commonTest/kotlin/bipack/BipackEncoderTest.kt
+++ b/src/commonTest/kotlin/bipack/BipackEncoderTest.kt
@@ -11,11 +11,11 @@ import kotlin.test.*
 data class Foobar1N(val bar: Int, val foo: Int = 117)
 
 @Serializable
-@ExtendableFormat
+@Extendable
 data class Foobar1(val bar: Int, val foo: Int = 117)
 
 @Serializable
-@ExtendableFormat
+@Extendable
 @SerialName("bipack.Foobar1")
 data class Foobar2(val bar: Int, val foo: Int, val other: Int = -1)
 
@@ -30,12 +30,12 @@ data class FoobarF2(val bar: Int, val foo: Int, val other: Int = -1)
 
 @Serializable
 @Framed
-@ExtendableFormat
+@Extendable
 data class FoobarF3(val bar: Int, val foo: Int, val other: Int = -1)
 
 @Serializable
 @Framed
-@ExtendableFormat
+@Extendable
 @CrcProtected()
 data class FoobarFP1(val bar: Int, val foo: Int, val other: Int = -1)