reserve i,j for Nat;

theorem
  for A,B being Matrix of REAL holds A--B=A+B
proof
  let A,B be Matrix of REAL;
  A+B = MXF2MXR ((MXR2MXF A)+(MXR2MXF --B)) by MATRIX_4:1;
  hence thesis by MATRIX_4:def 1;
end;
