class PulserList
{
  Node first;
  Node last;
  private Node pointer;
  private Node iterater;
  
  PulserList()
  {
    first = null;
    last = null;
    pointer = null;
    iterater = null;
  }
  
  boolean isEmpty()
  {
    return first == null;
  }    
  
  void add(Pulser p)
  {
    if ( first == null ) 
    {
      first = new Node(p);
      last = first;
      pointer = first;
    }
    else 
    {
      last.next = new Node(p);
      last.next.previous = last;
      last = last.next;
    }
  }
  
  void remove(Pulser p)
  {
    if ( isEmpty() ) 
    {
      return;
    }
    pointer = first;
    while ( pointer != null )
    {
      if ( pointer.puls == p )
      {
        if ( pointer.previous == null )
        {
          // p is the first element.
          first = pointer.next;
          if ( first != null ) first.previous = null;
        }
        else if ( pointer.next == null )
        {
          // p is the last element
          last = pointer.previous;
          last.next = null;
        }
        else
        {
          // p is in the middle somwhere
          pointer.previous.next = pointer.next;
          pointer.next.previous = pointer.previous;
        }
        break;
      }
      pointer = pointer.next;
    }
  }
  
  Pulser next()
  {
    if ( iterater == null )
    {
      iterater = first;
    }
    else
    {
      iterater = iterater.next;
    }
    return ( iterater != null ? iterater.puls : null );
  }
  
  void rewind()
  {
    iterater = null;
  }
  
  int length()
  {
    int i = 0;
    pointer = first;
    while ( pointer != null )
    {
      i++;
      pointer = pointer.next;
    }
    return i;
  }     
} 


class Node
{
  Pulser puls;
  Node next;
  Node previous;
  
  Node(Pulser p)
  {
    puls = p;
    next = null;
    previous = null;
  }
}