theorem Th58:
   x * (-y) = -(x*y) & (-x) * y = -(x*y) & (-x) * (-y) = x*y
proof
  (-x) * (-y) = - ( (-x)*y) by Lm8;
  then (-x) * (-y) = - -(x*y) by Lm8;
  hence thesis by Lm8;
end;
