How to automate file-opening (derived filename)?

[Danny Dorfman]

Hi!

I need to devise an e-lisp macro, that would do the following:
Assume I am now looking at a file named "/a/b/c/ZZZ/d/e.cpp", I'd like
the macro to open "/a/b/c/YYY/d/e.h" for me. The elements a,b,c,d,e
are unknown in advance, while elements ZZZ and YYY are a constant part
of the path.
How do I do this? (My understanding of e-lisp is pretty basic).

I am using GNU Emacs 21.2.1.

Regards,
Danny