public INumericExpression ParseNumericExpression(string expression)
 {
     lexer = new ExpressionLexer(expression);
     return((INumericExpression)Expression());
 }
 public IBooleanExpression ParseBooleanExpression(string expression)
 {
     lexer = new ExpressionLexer(expression);
     return((IBooleanExpression)Expression());
 }