fact: Definition rsubmx (A : 'M[R]_(m, n1 + n2)) := \matrix[rsubmx_key]_(i, j) A i (rshift n1 j).
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)