fact: Definition triad (p : seq G) := let adjp y := fband p (edge y) in [pred x | (2 < count adjp (orbit face x)) && ~~ (p \subset adj x)].
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq choice) mathcomp(fintype path fingraph) fourcolor(hypermap walkup geometry color chromogram)