Rewrite rules involving LHS lambda?

[Conal Elliott]

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
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