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 Th44:
  {f}++{g,h} = {f+g,f+h}
proof
  thus {f}++{g,h} = {f}++({g}\/{h}) by ENUMSET1:1
    .= ({f}++{g}) \/ ({f}++{h}) by Th41
    .= {f+g} \/ ({f}++{h}) by Th43
    .= {f+g} \/ {f+h} by Th43
    .= {f+g,f+h} by ENUMSET1:1;
end;
