/* Insert new_element after the cons-cell ptr_list */
void insert_after(void *new_element, cons_cell *ptr_list){
  cons_cell *new_cons_cell, *ptr_after;
 
  /* Make a new cons-cell around new_element */
  new_cons_cell= cons(new_element,NULL);

  /* Remember pointer to cons-cell after ptr_list */
  ptr_after = tail(ptr_list);

  /* Chain in the new_cons_cell */
  set_tail(ptr_list, new_cons_cell);

  /* ... and connect to rest of list */
  set_tail(new_cons_cell, ptr_after);
}