@@ -20,8 +20,9 @@
 abstract class Value
 {
 }
 
+///
 class IntValue : Value
 {
 	BigInt data;
 
@@ -28,8 +29,9 @@
 	mixin SimpleClass;
 	override string toString() const { return std.bigint.toDecimalString(cast(BigInt)data); }
 }
 
+///
 class StrValue : Value
 {
 	string data;
 
@@ -36,8 +38,9 @@
 	mixin SimpleClass;
 	override string toString() const { return data; }
 }
 
+///
 class FunValue : Value
 {
 	Value delegate(immutable LexPosition pos, string lay, Value[]) data;
 
@@ -45,13 +48,45 @@
 	alias data call;
 	override string toString() const { return sprintf!"(function:%s:%s)"(data.ptr,data.funcptr); }
 }
 
+///
 class UndValue : Value
 {
 	mixin SimpleClass;
 	override string toString() const { return "<undefined>"; }
 }
+
+/// Named Constructor for FunValue
+
+FunValue nativef(Value delegate(immutable LexPosition pos, Layer lay, Value[] args) dg)
+{
+	return new FunValue(dg);
+}
+
+/// Named Constructor for FunValue
+
+FunValue native(R,T...)(R delegate (T) dg)
+{
+	return nativef( delegate Value(immutable LexPosition pos, Layer lay, Value[] args) {
+		if( lay != "@v" )
+			throw genex!RuntimeException(pos, "only @v layer can call native function");
+		if( T.length != args.length )
+			throw genex!RuntimeException(pos, "argument number mismatch!");
+		T typed_args;
+		foreach(i, Ti; T)
+		{
+			typed_args[i] = cast(Ti) args[i];
+			if( typed_args[i] is null )
+				throw genex!RuntimeException(pos, sprintf!"type mismatch on the argument %d"(i+1));
+		}
+		try {
+			return dg(typed_args);
+		} catch( RuntimeException e ) {
+			throw e.pos is null ? new RuntimeException(pos, e.msg, e.file, e.line) : e;
+		}
+	});
+}
 
 /// Layer ID
 
 alias string Layer;
@@ -73,11 +108,24 @@
 			return;
 		data[i][lay] = v;
 	}
 
+	bool has(string i, Layer lay, in LexPosition pos=null)
+	{
+		if( i in data ) {
+			if( lay !in data[i] )
+				return false;
+			return true;
+		}
+		if( prototype is null )
+			return false;
+		return prototype.has(i, lay, pos);
+	}
+	
 	Value get(string i, Layer lay, in LexPosition pos=null)
 	{
 		if( i in data ) {
+			// [TODO] consider forwarding to proto also in this case
 			if( lay !in data[i] )
 				throw genex!RuntimeException(pos, sprintf!"variable %s is not set in layer %s"(i,lay));
 			return data[i][lay];
 		}