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
  {f,g}--{h} = {f-h,g-h}
proof
  thus {f,g}--{h} = {f,g}++{-h} by Th9
    .= {f-h,g-h} by Th44;
end;
