theorem Th22:
  p '&' q <> r => s
proof
  p '&' q = <*2*>^(p^q) by FINSEQ_1:32;
  then r => s = <*1*>^(r^s) & (p '&' q).1 = 2 by FINSEQ_1:32,41;
  hence thesis by FINSEQ_1:41;
end;
