It's no problem to find `bar.gpi' from `Program Humpta'. However `Module Foo
Implementation' does not explicitly import `Bar', so it does not know where to
find those `(some, identifiers, listed, here)'. To locate them, it looks at
the name of the module, `Foo', looks up `foo.gpm' where `bar' is listed. Now
it can find `bar.gpi' - voila!
If it looks weird for you, never mind, it does so for me as well. You don't
need to worry about all this unless you want to change from UCSD-style Units
to EP-style Modules. ;-)
Just an idea (since I don't like to have too many files around): it might be
possible to have one file (with the unit name = file name) in the case of
units and two files (one with the module name (= file name?) and one with
the interface name) in the case of modules. Not urgent, of course...