public class UnknownOpException extends Exception
{
    public UnknownOpException()
    {
        super("UnknownOpException"); 
    }
   
    public UnknownOpException(char op)
    {
        super(op + " is an unknown operator."); 
    }
   
    public UnknownOpException(String message)
    {
        super(message); 
    }
}