reserve a for set;

theorem Th21:
  for L being lower-bounded non empty Poset
  for I being Ideal of L holds Bottom L in I
proof
  let L be lower-bounded non empty Poset;
  let I be Ideal of L;
  set x = the Element of I;
  Bottom L <= x by YELLOW_0:44;
  hence thesis by WAYBEL_0:def 19;
end;
