interface queue 
  {
    queue();                     // constructor.
    virtual ~queue();            // destructor.
  
    void reset();                // reset queue.
  
    int reportsize(histogram* h,double interval = 0);
    int reportwaiting(histogram* h,double interval = 0,int qp=0); 
    int reportwaitingall(histogram* h,double interval = 0); 
  
    int prepend(event* e);      // add to front.
    int append(event* e);       // add to back.
  
    event* removefront();       // remove front.
    event* front();             // return front.
    event* removeback();        // remove back.
    event* back();              // return back.
  
    int extract(event* e);      // extract e.
  
    int terminatefront();       // terminate front
    int terminateback();        // terminate back
  
    int cancel();               // cancel specified events
  
    int size();                 // return size.
    int empty();                // is empty ?
  
    friend ostream& operator<<(ostream& os,queue& q);
  };