proposition: Lemma deg_Kn n (x : 'K_n.+1) : n <= #|N(x)|.
proof: Proof. have E : N(x) =i [pred y in 'K_n.+1 | y != x] by move=> y; rewrite !inE eq_sym. by rewrite -ltnS -[#|_|.+1]add1n (eq_card E) -cardD1x // card_ord. Qed.
imports: Coq(Setoid CMorphisms) mathcomp(all_ssreflect) GraphTheory(edone preliminaries bij digraph) GraphTheory(set_tac)