reserve L for Lattice,
  p,p1,q,q1,r,r1 for Element of L;
reserve x,y,z,X,Y,Z,X1,X2 for set;
reserve H,F for Filter of L;

theorem Th16:
  p in <.p.) & p "\/" q in <.p.) & q "\/" p in <.p.)
proof
  p [= p "\/" q by LATTICES:5;
  hence thesis;
end;
