proposition: Lemma switchC x y f : switch x y f =1 switch y x f.
proof: Proof. case: (altP (x =P y)) => [<-|xDy]; rewrite ?switchxx //= ?eqxx. move => z /=. do ! (case (z =P _) => [->|] //); by rewrite (negbTE xDy). Qed.
imports: mathcomp(all_ssreflect) GraphTheory(edone preliminaries bij) fourcolor(hypermap geometry cfmap jordan color coloring combinatorial4ct) fourcolor(walkup)