theorem Th16:
  s1 == s2 implies push(e,s1) == push(e,s2)
  proof
    assume |.s1.| = |.s2.|;
    hence |.push(e,s1).| = <*e*>^|.s2.| by Th8 .= |.push(e,s2).| by Th8;
  end;
