diff --git a/include/expr.cpp b/include/expr.cpp
index c1a7d56cf..e3a4b1236 100755
--- a/include/expr.cpp
+++ b/include/expr.cpp
@@ -194,10 +194,10 @@ TExpression::operator const char*()
 void TExpression::print_on(ostream& out) const       
 { out << _original; }
 
-void TExpression::evaluate_user_func(const char * name, int nparms, TStack & stack, TTypeexp  type) const
+void TExpression::evaluate_user_func(int index, int nparms, TStack & stack, TTypeexp  type) const
 
 {                
-  NFCHECK("Unknown function : %s ", name);
+  CHECKD(FALSE, "Unknown function ", index);
   for ( int i = nparms ; i > 0; i--)
     stack.pop();
   stack.push(ZERO);
@@ -445,8 +445,9 @@ void TExpression::eval()
     case _userfunc: 
       {
         const int nparms = (int) ((real &) evalstack.pop()).integer();
+        const int index = atoi(instr.string());
         
-        evaluate_user_func(instr.string(), nparms, evalstack, type);
+        evaluate_user_func(index, nparms, evalstack, type);
       }
       break;
     case _sqrt:
@@ -867,9 +868,19 @@ TCodesym TExpression::__factor(TCodesym startsym)
         if (sym == _rpar)
         {
           _code.add(_number, real(nparms));
-          _code.add(startsym, val);
-          _user_func_defined = TRUE;                      
-          sym = __gettoken(TRUE);
+          const int index = parse_user_func((const char *) val.string(), nparms);
+          if (index < 0) 
+          {
+            strcpy(_tok, (const char *) val.string());
+            sym = _invalid;
+          }
+          else
+          {                     
+            val.set(format("%d", index));
+            _code.add(startsym, val);
+            _user_func_defined = TRUE;                      
+            sym = __gettoken(TRUE);
+          }  
         }
       }
     }
diff --git a/include/expr.h b/include/expr.h
index f3c8a7dbb..766d6c8d4 100755
--- a/include/expr.h
+++ b/include/expr.h
@@ -416,7 +416,8 @@ protected:
 protected: // TObject
   // @cmember Stampa l'espressione su <p out> (serve per implementare l'insertore)
   virtual void print_on(ostream& out) const ;
-  virtual void evaluate_user_func(const char * name, int nparms, TStack & stack, TTypeexp type) const ;
+  virtual void evaluate_user_func(int index, int nparms, TStack & stack, TTypeexp type) const;
+  virtual int parse_user_func(const char * name, int nparms) const { return -1; }
 
   // @access Public Member
 public:
@@ -458,6 +459,8 @@ public:
   // @cmember Setta l'espressione e la compila (ritorna il risultato della compilazione)
   bool set(const char* expression, TTypeexp type = _numexpr);
   const char* last_compiled_token() const;
+  // @cmember Ritorna l'espressione originale 
+  const char * string() const { return _original; }
 
   // @cmember Costruttore (assegna l'estressione e il suo tipo)
   TExpression(const char* expression, TTypeexp type = _numexpr);