ons which use the Parse library may write their own functions. Whenever the formatter encounters a function name which it does not recognize, it calls the application to get an ID for the function. When the evaluator needs to evaluate that function, it calls the application, passing the arguments and the function ID. The application should return a single value. If it cannot produce a value, it should return an error code. The error codes are described in <A HREF="#Parse_b.htm_26281"> Evaluating a Token Seq