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;
