fact: Definition lker aT rT (f : 'Hom(aT, rT)) := mx2vs (kermx (f2mx f)).
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq choice) mathcomp(fintype bigop finfun tuple) mathcomp(ssralg matrix mxalgebra zmodp)