Make for better error messages when a clone fails

include/chaiscript/language/chaiscript_engine.hpp

@@ -166,6 +166,8 @@ namespace chaiscript
 
             engine.add(fun(boost::function<std::string (Boxed_Value)>(boost::bind(&chaiscript::type_name, boost::ref(engine), _1))),
                 "type_name");
+            engine.add(fun(boost::function<bool (const std::string &)>(boost::bind(&Eval_Engine::function_exists, boost::ref(engine), _1))),
+                "function_exists");
 
             engine.add(vector_type<std::vector<Boxed_Value> >("Vector"));
             engine.add(string_type<std::string>("string"));

include/chaiscript/language/chaiscript_prelude.hpp

@@ -13,7 +13,7 @@
 
 #define chaiscript_prelude CODE_STRING(\
 def new(x) { eval(type_name(x))(); } \
-def clone(x) { eval(type_name(x))(x); } \
+def clone(x) : function_exists(type_name(x))  { eval(type_name(x))(x); } \
 # to_string for Pair()\n\
 def to_string(x) : call_exists(first, x) && call_exists(second, x) { \
   "<" + x.first.to_string() + ", " + x.second.to_string() + ">"; \