fact: Definition sewr xr := match in_bGr xr with | inleft u => sewd (hrd (sval u) xr) | inright b'xr => sewr_r b'xr end.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq choice) mathcomp(fintype path fingraph) fourcolor(hypermap geometry color patch)