proposition: Lemma setXb_0 : productb emptyset = singleton emptyset.
proof: Proof. have h:= fgraph_set0. apply: set1_pr. by apply/setXb_P;split => //; rewrite domain_set0 => i /in_set0. by move => z /setXb_P [_]; rewrite domain_set0; move/domain_set0_P. Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat)