Fixpoint fcat' (s1 : fmap) (s2 : seq (K * V)) {struct s2} : fmap :=
if s2 is (k, v)::t then fcat' (ins k v s1) t else s1.
Definition fcat s1 s2 := fcat' s1 (seq_of s2).

Lemma fnd_fcat k s1 s2 :
fnd k (fcat s1 s2) =
if k \in supp s2 then fnd k s2 else fnd k s1.