/* Delete the element after ptr_list. Assume as a
   precondition that there exists an element after ptr_list */
void delete_after(cons_cell *ptr_list){  
  cons_cell *ptr_after, *ptr_dispose;

  /* cons-cell to delete later */
  ptr_dispose = tail(ptr_list);

  /* The element to follow ptr_list */
  ptr_after = tail(ptr_dispose);

  /* Mutate the tail of ptr_list */
  set_tail(ptr_list, ptr_after);

  /* Free storage - only one cons-cell */
  free(ptr_dispose);
}