reserve i,j for Nat;

theorem Th46:
  for K being Ring,M1,M2,M3 being Matrix of K st len M1=len M2 &
len M2=len M3 & width M1=width M2 & width M2 = width M3 holds - M1 - M2 - M3 =
  - M1 - M3 - M2
proof
  let K be Ring,M1,M2,M3 be Matrix of K;
A1: len (-M1)=len M1 & width (-M1)=width M1 by MATRIX_3:def 2;
  assume
  len M1=len M2 & len M2=len M3 & width M1=width M2 & width M2 = width M3;
  hence thesis by A1,Th16;
end;
