I'm trying to use .PRG's to store my class definitions, and I'm having a bit of a problem.

I created a base class named "Abc" and put it in a program named FIRST.PRG:

DEFINE CLASS Abc AS CUSTOM
FUNCTION Test1()
RETURN "Abc.Test1()"
ENDFUNC
ENDDEFINE

Then I created a second program SECOND.PRG:

DEFINE CLASS Def AS Abc
ENDDEFINE

Now how can I get the second class to know that the parent class Abc exists?