theorem
  <.p.) = <.q.) implies p = q
proof
  assume
A1: <.p.) = <.q.);
  then q in <.p.);
  then
A2: p [= q by FILTER_0:15;
  p in <.q.) by A1;
  then q [= p by FILTER_0:15;
  hence thesis by A2,LATTICES:8;
end;
