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,i} = {f-h,f-i,g-h,g-i}
proof
  thus {f,g}--{h,i} = {f,g}++{-h,-i} by Th10
    .= {f-h,f-i,g-h,g-i} by Th45;
end;
