docs: public void replace( Token original , Token target  ) {
          if( first == original )
              first = target;
          if( last == original )
              last = target;
  
          target.next = original.next;
          target.previous = original.previous;
  
          if( original.next != null )
              original.next.previous = target;
          if( original.previous != null )
              original.previous.next = target;
  
          original.next = original.previous = null;
      }