fact: Definition two_option_void: bij (option (void+void) + option (void+void)) (option (option ((void+void)+void))).
imports: HB(structures) Coq(Setoid Morphisms) mathcomp(all_ssreflect) GraphTheory(edone finite_quotient preliminaries bij equiv) GraphTheory(setoid_bigop structures pttdom mgraph mgraph2 rewriting)