fact: Definition dsubmx (A : 'M[R]_(m1 + m2, n)) := \matrix[dsubmx_key]_(i, j) A (rshift m1 i) 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)