// A direction; one of 'north', 'south', 'east' and 'west'.

class Direction {

    public String direction;
    
    public Direction(String s) {
    
        if (!s.equals("north") && !s.equals("south") &&
            !s.equals("east") && !s.equals("west")) {
            System.err.println(s + " is an invalid direction.  Must be " +
                "'north', 'south', 'east' or 'west'");
            System.exit(0);
        }
        
        direction = s;
        
    }
    
    public boolean equals(Direction d) {
        return d.equals(direction);
    }
    
    public boolean equals(String s) {
        return s.equals(direction);
    }
    
    public String toString() { return direction; }
    
    public Direction opposite() {
        Direction d = null;
        if (direction.equals("north")) {
            d = new Direction("south");
        } else if (direction.equals("south")) {
            d = new Direction("north");
        } else if (direction.equals("east")) {
            d = new Direction("west");
        } else if (direction.equals("west")) {
            d = new Direction("east");
        }
        
        return d;
    }
    
}