fact: Definition drule21 := Part $ 6- $ 6 $[5] 7 $ 6+ $ * $ 5 $ 5 $.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq choice) mathcomp(fintype path fingraph) mathcomp(bigop order ssralg ssrnum ssrint) fourcolor(hypermap geometry part)