fact: Variable similar_PlusC : forall c1 c2, similar (Plus c1 c2) (Plus c2 c1).
imports: Coq(RelationClasses) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq fintype) mathcomp(bigop path) RegLang(languages) RegexpBrzozowski(glue gfinset regexp finite_der)