reserve i,j for Nat;

theorem
  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;
A1: M1+(M2 + -M2)=M1+(M2-M2);
  assume len M1=len M2 & width M1=width M2;
  hence thesis by A1,Th20;
end;
