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 Th53:
  {a,b}++{s,t} = {a+s,a+t,b+s,b+t}
proof
  thus {a,b}++{s,t} = ({a}\/{b})++{s,t} by ENUMSET1:1
    .= ({a}++{s,t}) \/ ({b}++{s,t}) by Th48
    .= {a+s,a+t} \/ ({b}++{s,t}) by Th52
    .= {a+s,a+t} \/ {b+s,b+t} by Th52
    .= {a+s,a+t,b+s,b+t} by ENUMSET1:5;
end;
