Rewrite rules involving LHS lambda?

Is there a written explanation and/or examples of rewrite rules involving a LHS lambda? Since rule matching is first-order, I'm wondering how terms with lambda are matched on the LHS and substituted into on the RHS. For instance, I want to restructure a lambda term as follows:

foo (\ x -> fmap u v) = bar (\ x -> u) (\ x -> v)

My intent is that the terms `u` and `v` may contain `x` and that whatever variable name is actually used in a term being rewritten is preserved so as to re-capture its occurrences on the RHS. When I write this sort of rule, I get warnings about `x` being defined but not used. Thanks, -- Conal