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/*
    This file is part of JLog.

    Created by Glendon Holst for Alan Mackworth and the 
    "Computational Intelligence: A Logical Approach" text.
    
    Copyright 1998, 2000, 2002 by University of British Columbia and 
    Alan Mackworth.
    
    This notice must remain in all files which belong to, or are derived 
    from JLog.
    
    Check  or 
     for further information
    about JLog, or to contact the authors.
     
    JLog is free software; you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
    the Free Software Foundation; either version 2 of the License, or
    (at your option) any later version.

    JLog is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    GNU General Public License for more details.

    You should have received a copy of the GNU General Public License
    along with JLog; if not, write to the Free Software
    Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
    URLs:  or 
*/
//#########################################################################
//	iGoalStack
//#########################################################################

package ubc.cs.JLog.Foundation;

import java.lang.*;
import java.util.*;

/**
* iGoalStack is the goal stack interface.
* A goal stack contains items of type jGoal that typically represent either 
* proved goals or goals that need to be proved.
*  
* @author       Glendon Holst
* @version      %I%, %G%
*/
public interface iGoalStack
{
 /**
  * Determines if the stack does not contain goals.
  *
  * @return 		true if the stack is empty, 
  * 			false otherwise.
  */
 public boolean 	empty();
 
 /**
  * Returns and removes the top goal of the stack.
  * Do not pop an empty stack.
  *
  * @return 		the top jGoal of the stack.
  */
 public jGoal 		pop();
 
 /**
  * Returns the top goal of the stack, without removing that goal.
  * Do not peek an empty stack.
  *
  * @return 		the top jGoal of the stack.
  */
 public jGoal 		peek();
 
 /**
  * Returns the nth goal on the stack, without removing that goal.
  * Do not peek an empty stack. 0 is the top element.
  *
  * @return 		the nth jGoal of the stack.
  */
 public jGoal 		peekn(int n);
 
 /**
  * Places the provided goal item as the top element of the stack.
  *
  * @param item 	the goal to add to the stack top.
  * @return 		the top jGoal of the stack, which is the 
  *			provided item.
  */
 public jGoal 		push(jGoal item);
 
 /**
  * Removes all goals above the specified item.  item 
  * becomes the top goal.
  *
  * @param item 	the goal to truncate the stack at. item becomes
  *			the stack top. item must be in this stack
  *			instance.
  * @return 		the top jGoal of the stack, which is the 
  *			provided item.
  */
 public jGoal 		cut(jGoal item);
};