fact: Definition castmx m' n' (eq_mn : (m = m') * (n = n')) A : 'M_(m', n') := let: erefl in _ = m' := eq_mn.1 return 'M_(m', n') in let: erefl in _ = n' := eq_mn.2 return 'M_(m, n') in A.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat seq choice) mathcomp(fintype finfun bigop finset fingroup perm order) mathcomp(div prime binomial ssralg countalg finalg zmodp)