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 Th28:
  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 by XCMPLX_1:201;
end;
