proposition: Lemma sumf_inj : injective sumf.
proof: Proof. move => [u|u] [v|v] //= []; by [ move/inj_f1->| move/inj_f2 ->]. Qed.
imports: mathcomp(all_ssreflect) GraphTheory(edone preliminaries bij) fourcolor(hypermap geometry cfmap jordan color coloring combinatorial4ct) fourcolor(walkup)