fact: Definition embr_face x := oapp (embdd \o edge) (face x) (pick (bGd_proj (edge x))).
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq choice) mathcomp(fintype path fingraph) fourcolor(hypermap geometry color chromogram coloring patch) fourcolor(snip revsnip kempe birkhoff contract)