theorem Th23:
  inferior_setsequence B is non-descending
proof
  now
    let n be Nat;
    (inferior_setsequence B).n = (inferior_setsequence B).(n+1) /\ B.n by Th21;
    hence (inferior_setsequence B).n c= (inferior_setsequence B).(n+1) by
XBOOLE_1:17;
  end;
  hence thesis by PROB_2:7;
end;
