Function to check whether a class has been defined
The CLASS_EXISTS() function can be used to check whether a class has been defined. CLASS_EXISTS() will return .T. (true) if the class named <expC> has been defined and .F. (false) if it has not been defined. Classes are defined using the DEFINE CLASS command. Class libraries are loaded using SET CLASSLIB.
set classlib to myclasses1 ? set([classlib]) list classes ? class_exists("mybutton1")