reserve T for non empty TopSpace,
  X,Z for Subset of T;
reserve x,y for Element of OpenClosedSet(T);
reserve x,y,X for set;
reserve BL for non trivial B_Lattice,
  a,b,c,p,q for Element of BL,
  UF,F,F0,F1,F2 for Filter of BL;

theorem Th19:
  for F st F is being_ultrafilter holds a "\/" b in F iff a in F or b in F
proof
  let F;
  assume F is being_ultrafilter;
  then F is prime by FILTER_0:45;
  hence thesis by FILTER_0:def 5;
end;
