>At the time of the subclass definition, the upper class must be in scope by a SET PROCEDURE TO MyClass.prg. You must create a new prg (assuming you don't want to change the original one) in which you just redefine what needs to be changed.
>
>For instance, this being MyClass.prg
>
>

>DEFINE CLASS SomeClass AS Custom
>
>  FUNCTION Method_One
>    LPARAMETERS Parm1 AS Number, Parm2 AS Number
>    RETURN m.Parm1 * m.Parm2
>  ENDFUNC
>
>  FUNCTION Method_Two
>    LPARAMETERS Parm1 AS Number, Parm2 AS Number
>    RETURN m.Parm1 / m.Parm2
>  ENDFUNC
>
>END DEFINE
>

>
>you may write a different MySubClass.prg with
>
>

>DEFINE CLASS SomeSubClass AS SomeClass
>
>  FUNCTION Method_Two
>    LPARAMETERS Parm1 AS Number, Parm2 AS Number
>    RETURN IIF(m.Parm2 = 0, .NULL., m.Parm1 / m.Parm2)
>  ENDFUNC
>
>END DEFINE
>

That's not necessary. Use the OF keyword:

DEFINE CLASS SomeSubClass AS SomeClass of MyClass.PRG

Tamar