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 Th11:
  a in A iff -a in --A
proof
  thus a in A implies -a in --A;
  assume -a in --A;
  then ex c st -c = -a & c in A;
  hence thesis;
end;
