How to pass unknown type data as parameter to function?

How to pass unknown type data as parameter to function?

F.e. I want to implement inc function:
FUNCTION inc RETURNS INT (INPUT-OUTPUT i AS INT, AddExpression AS INT):
i = i + AddExpression.
END FUNCTION.
to use it like this:
inc(tt-data.qty,1).
I didn't found how to override my function for DEC data type or how to
combine both in one. If it possible I also want my function to deal with
CHAR - kind of ADD-ENTRY. Maybe this basic functions are already
implemented by someone? Something like this STLib on OEHive.