reserve L for Lattice,
  p,q,r for Element of L,
  p9,q9,r9 for Element of L.:,
  x, y for set;
reserve I,J for Ideal of L,
  F for Filter of L;

theorem Th22:
  p in I implies p"/\"q in I & q"/\"p in I
proof
  p"/\"q [= p by LATTICES:6;
  hence thesis by Th21;
end;
