Q: adding to compose key mappings?

[D. R. Evans]

Does anyone know where the file is that is used to define the mappings
for the compose key (meaning, what file tells the system that when I
hit compose-'-e then I really want an é?).

I want to add some additional sequences for some unicode characters
that right now I cannot easily produce when I am typing (such as the
musical flat symbol, for example).