proposition: Fact ffun_addC : commutative ffun_add.
proof: Proof. by move=> f1 f2; apply/ffunP=> a; rewrite !ffunE addrC. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat div seq) mathcomp(choice fintype finfun bigop prime binomial)