fact: Definition ex2C (T : Type) (P Q : T -> Prop) : @ex2 T P Q <-> @ex2 T Q P.
imports: mathcomp(all_ssreflect ssralg fingroup perm matrix) mathcomp(all_algebra vector reals normedtype) mathcomp(mathcomp_extra boolp) mathcomp(Rstruct) ssrR logb realType_ext ssr_ext ssralg_ext bigop_ext