theorem Th16:
  for a holds { F :F is being_ultrafilter & a in F} c= ultraset BL
proof
  let a;
  let x be object;
  assume x in { F :F is being_ultrafilter & a in F};
  then ex UF st ( UF = x)&( UF is being_ultrafilter)&( a in UF);
  hence thesis;
end;
