Index: index.dd
==================================================================
--- index.dd
+++ index.dd
@@ -601,10 +601,12 @@
 
 $(SECTION 外部とのやりとり, $(SECBODY
   $(TABLE
     $(TR $(TH print) $(TD (a)) $(TD a を文字列化標準出力に改行付きで表示))
     $(TR $(TH argv) $(TD ) $(TD スクリプトに渡された引数文字列のconsリスト))
+    $(TR $(TH gensym) $(TD ()) $(TD エセgensym。変数名として他とかぶらなそうな文字列を返します))
+    $(TR $(TH rand) $(TD (n)) $(TD 0 以上 n 未満の自然数をランダムに生成します))
   )
 ))
 <br />
 
 $(SECTION データ型判定, $(SECBODY

Index: polemy/runtime.d
==================================================================
--- polemy/runtime.d
+++ polemy/runtime.d
@@ -7,13 +7,15 @@
  */
 module polemy.runtime;
 import polemy._common;
 import polemy.layer;
 import polemy.failure;
-import polemy.value;
+import polemy.fresh;
+import polemy.value;
 import polemy.eval;
 import std.stdio;
+import std.random;
 
 /// enroll the native implementations of primitive functions
 
 void enrollRuntimeLibrary( Evaluator e )
 {
@@ -62,11 +64,13 @@
 		(Value v){return new IntValue(cast(UndefinedValue)v !is null);} );
 	e.addPrimitive("_istable", ValueLayer,
 		(Value v){return new IntValue(cast(Table)v !is null);} );
 	// table
 	e.addPrimitive(".", ValueLayer, (Table t, StrValue s){
-		return (t.has(s.data, ValueLayer) ? t.get(s.data, ValueLayer) : new UndefinedValue);
+		if( t.has(s.data, ValueLayer) )
+			return t.get(s.data, ValueLayer);
+		throw genex!RuntimeException(text("table do not have the field ",s));
 	});
 	e.addPrimitive(".?", ValueLayer, (Table t, StrValue s){
 		return new IntValue(t.has(s.data, ValueLayer));
 	});
 	e.addPrimitive(".=", ValueLayer, (Table t, StrValue s, Value v){
@@ -75,8 +79,13 @@
 		return t2;
 	});
 	e.addPrimitive("{}", ValueLayer, (){
 		return new Table;
 	});
-	// IO
+	// IO and others
 	e.addPrimitive("print", ValueLayer, (Value a){ writeln(a); return new IntValue(0); });
+	e.addPrimitive("gensym", ValueLayer, (){ return new StrValue(freshVarName()); });
+	auto rand = Mt19937(unpredictableSeed);
+	e.addPrimitive("rand", ValueLayer, (IntValue n){
+		return new IntValue( uniform(0,cast(int)n.data.toInt(),rand) );
+	});
 }