proposition: Lemma csum_trivial1 x f: domain f = singleton x -> csum f = cardinal (Vg f x).
proof: Proof. rewrite/csum /disjointU/disjointU_fam => ->. rewrite /unionb Lgd setUf_1 LgV //; aw; trivial; exact:(set1_1 x). Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat)