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
  {a,b}--{s,t} = {a-s,a-t,b-s,b-t}
proof
  thus {a,b}--{s,t} = {a,b}++{-s,-t} by Th20
    .= {a-s,a-t,b-s,b-t} by Th53;
end;
