private double _evaluate(int beg,int end) throws NumberFormatException, ArithmeticException {
  return _evaluate(beg,end,0.0,OPERAND,getOperator('='));
}

private double _evaluate ( int beg , int end ) throws NumberFormatException , ArithmeticException { return _evaluate ( beg , end , 0.0 , OPERAND , getOperator ( '=' ) ) ; }
evaluate a complete ( sub - ) expression .
