reserve w, w1, w2 for Element of ExtREAL;
reserve c, c1, c2 for Complex;
reserve A, B, C, D for complex-membered set;
reserve F, G, H, I for ext-real-membered set;
reserve a, b, s, t, z for Complex;
reserve f, g, h, i, j for ExtReal;
reserve r for Real;
reserve e for set;

theorem Th18:
  -- (A \+\ B) = (--A) \+\ (--B)
proof
  thus -- (A \+\ B) = --(A\B) \/ --(B\A) by Th15
    .= --A \ --B \/ --(B\A) by Th17
    .= (--A) \+\ --B by Th17;
end;
