Definition ptd_stack_compat (ptd:pt_dot) (stack:stack): Prop :=
  match ptd with
    | Reduce_ptd ptlz => ptlz_stack_compat ptlz stack
    | Shift_ptd _ _ _ ptlz => ptlz_stack_compat ptlz stack
  end.Fixpoint build_pt_dot {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)
    :pt_dot :=
  match ptl in parse_tree_list hole_symbs hole_word hole_sems
    return ptl_zipper hole_symbs hole_word hole_sems -> _
  with
    | Nil_ptl => fun ptlz =>
      Reduce_ptd ptlz
    | Cons_ptl pt ptl' =>
      match pt in parse_tree hole_symb hole_word hole_sem
        return ptl_zipper (hole_symb::_) (hole_word++_) (hole_sem,_) -> _
      with
        | Terminal_pt term sem => fun ptlz =>
          Shift_ptd term sem ptl' ptlz
        | Non_terminal_pt ptl'' => fun ptlz =>
          build_pt_dot ptl''
            (Non_terminal_pt_ptlz (Cons_ptl_ptz ptl' ptlz))
      end
  end ptlz.

Lemma build_pt_dot_buffer: 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), ptd_buffer (build_pt_dot ptl ptlz) = hole_word ++ ptlz_buffer ptlz.