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+M2)=M1+-M2
proof
  let K be Ring,M1,M2 be Matrix of K;
A1: len (-M1)=len M1 & width (-M1)=width M1 by MATRIX_3:def 2;
  assume len M1=len M2 & width M1=width M2;
  then -(-M1+M2)=--M1+-M2 by A1,Th12;
  hence thesis by Th1;
end;
