theorem Th21:
  a <> b implies ex F st F in F_primeSet(L)
proof
  assume a <> b;
  then not a [= b or not b [= a by LATTICES:8;
  then (ex F st F in F_primeSet(L) & not b in F & a in F) or ex F st F in
  F_primeSet(L) & not a in F & b in F by Th20;
  hence thesis;
end;
