Program Fixpoint pop_ptlz {hole_symbs hole_word hole_sems}
  (ptl:parse_tree_list hole_symbs hole_word hole_sems)
  (ptlz:ptl_zipper hole_symbs hole_word hole_sems):
    { word:_ & { sem:_ &
      (pt_zipper (NT (prod_lhs (ptlz_prod ptlz))) word sem *
       parse_tree (NT (prod_lhs (ptlz_prod ptlz))) word sem)%type } } :=
  match ptlz in ptl_zipper hole_symbs hole_word hole_sems
    return parse_tree_list hole_symbs hole_word hole_sems ->
    { word:_ & { sem:_ &
      (pt_zipper (NT (prod_lhs (ptlz_prod ptlz))) word sem *
       parse_tree (NT (prod_lhs (ptlz_prod ptlz))) word sem)%type } }
  with
    | @Non_terminal_pt_ptlz prod word sem ptz => fun ptl =>
      let sem := uncurry (prod_action prod) sem in
      existT _ word (existT _ sem (ptz, Non_terminal_pt ptl))
    | Cons_ptl_ptlz pt ptlz' => fun ptl =>
      pop_ptlz (Cons_ptl pt ptl) ptlz'
  end ptl.

Lemma pop_ptlz_cost: forall hole_symbs hole_word hole_sems (ptl:parse_tree_list hole_symbs hole_word hole_sems) (ptlz:ptl_zipper hole_symbs hole_word hole_sems), let 'existT _ word (existT _ sem (ptz, pt)) := pop_ptlz ptl ptlz in ptlz_cost ptlz = ptz_cost ptz.