proposition: Lemma simple_fct a b A B (f := singleton (J a b)): inc a A -> inc b B -> [/\ fgraph f, domain f = singleton a, range f = singleton b, sub (domain f) A & (sub (range f) B /\ Vg f a = b) ].
proof: Proof. move => aA bB. move: (fgraph_set1 a b) => [ha hb hc]; rewrite hb hc; split => //. + by move => t /set1_P ->. + split; [by move => t /set1_P -> | symmetry ]. by move:(pr2_V ha (set1_1 (J a b))); aw. Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat)