Need help writing file-visiting macro

[Roy Smith]

I'm working in a software development system which has classes defined
in foo.mdl files, with the corresponding implementation code in
foo_Impl.c files I want to write a macro or function which lets you
flip back and forth between the two.  If I'm looking at a .mdl file, I
want it to construct the corresponding _Impl.c filename and visit that
file.  Is that possible?

I'm using GNU Emacs 21.3.1.