fact: Definition inset r := let: Grect x0 x1 y0 y1 := r in Grect (x0 + 1) (x1 - 1) (y0 + 1) (y1 - 1).
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq path) mathcomp(div choice order ssralg ssrnum ssrint intdiv) fourcolor(hypermap)