proposition: Lemma not_fnd V (f : {fmap K -> V}) (k : K) : k \notin f -> f.[? k] = None.
proof: Proof. by case: fndP. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrnat eqtype ssrfun seq) mathcomp(choice path finset finfun fintype bigop)