fact: Definition block_mx Aul Aur Adl Adr : 'M_(m1 + m2, n1 + n2) := col_mx (row_mx Aul Aur) (row_mx Adl Adr).
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)