fact: Definition mcorner m q : nat := let m'q i := q - iter i ccw 0 \notin m in (m'q 2 + (m'q 1 + m'q 3).*2)%N.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq choice) mathcomp(path order ssralg ssrnum ssrint div intdiv) fourcolor(grid)