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 Th28:
  q in (.p.> iff q [= p
proof
  q in (.p.> iff ex r st q = r & r [= p;
  hence thesis;
end;
