fact: Definition lcutmx (A : 'M_(m, n1 + n2)):= \matrix_(i < m, j < n1) (A i (lshift n2 j) : R).
imports: ssreflect ssrbool ssrfun eqtype ssrnat seq choice fintype finfun bigops ssralg groups perm zmodp morphisms Ring RingClass bigopsClass Setoid Morphisms