fact: Definition mxdirect_def m n T of phantom 'M_(m, n) (unwrap (mxsum_val T)) := \rank (unwrap T) == unwrap (mxsum_rank T).
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat seq choice) mathcomp(fintype finfun bigop finset fingroup perm order) mathcomp(div prime binomial ssralg finalg zmodp matrix)