theorem
  t*z = t+z implies t=z
  proof
    assume t*z = t+z; then
    0 = (t-1)*(z-1) - 1; then
    (t-1 = 1 & z-1 = 1) or (t-1 = -1 & z-1 = -1) by INT_1:9;
    hence thesis;
  end;
