Section Map.
Variables (K : ordType) (U V : Type) (f : U -> V).
Definition mapf' (m : seq (K * U)) : seq (K * V) :=
map (fun kv => (key kv, f (value kv))) m.

Lemma mapf_fcat s1 s2 : mapf (fcat s1 s2) = fcat (mapf s1) (mapf s2).