reserve i,j for Nat;

theorem Th21:
  for K being Ring,M1,M2 being Matrix of K st len M1=len M2 &
  width M1=width M2 holds M1 = M1 + M2 - M2
proof
  let K be Ring,M1,M2 be Matrix of K;
  assume
A1: len M1=len M2 & width M1=width M2;
  hence M1+M2-M2= M1+(M2-M2) by MATRIX_3:3
    .=M1 by A1,Th20;
end;
