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 Th10:
  --{f,g} = {-f,-g}
proof
  thus --{f,g} = --({f}\/{g}) by ENUMSET1:1
    .= --{f} \/ --{g} by Th5
    .= {-f} \/ --{g} by Th9
    .= {-f} \/ {-g} by Th9
    .= {-f,-g} by ENUMSET1:1;
end;
