How do I rewrite a expression in terms of other expressions? (Using just algebraic transformations.)

I'm look for something like algsubs. algsubs does not work for some non-trivial cases: I get the error "cannot compute degree of pattern in...". Playing a bit with these expresions (replacing denominators, and some subexpressions), I can transform in various steps, applying algsubs, to get the desire result, but I don't know how do this automatically.

Hello,

How can I use assign(...) inside in proc in order to affect the local variables and not the globals.

Thanks