public Sequence eval(DocumentSet docs, Sequence contextSequence, Item contextItem)
public Sequence eval(Sequence contextSequence, Item contextItem)
	public Sequence eval(DocumentSet docs, Sequence contextSequence, Item contextItem) 	throws XPathException {		if(contextItem != null)			contextSequence = contextItem.toSequence();		if(getLength() == 0)			throw new XPathException("unary expression requires an operand");		NumericValue value = (NumericValue)			getExpression(0).eval(docs, contextSequence).convertTo(Type.NUMBER);		if(mode == Constants.MINUS)			return value.negate();		else			return value;	}