fact: Definition pastemx (A1 : 'M_(m, n1)) (A2 : 'M_(m, n2)) := \matrix_(i < m, j < n1 + n2) (match split j with inl j1 => A1 i j1 | inr j2 => A2 i j2 end : R).
imports: ssreflect ssrbool ssrfun eqtype ssrnat seq choice fintype finfun bigops ssralg groups perm zmodp morphisms Ring RingClass bigopsClass Setoid Morphisms