Sequence inner = getArgument(0).eval(context, docs, contextSequence, contextItem);
Sequence inner = getArgument(0).eval(docs, contextSequence, contextItem); if(inner.getLength() == 0) return DoubleValue.ZERO;
    public Sequence eval(StaticContext context, DocumentSet docs, Sequence contextSequence,    	Item contextItem) throws XPathException {		double sum = 0.0;		Sequence inner = getArgument(0).eval(context, docs, contextSequence, contextItem);		Item next;		for(SequenceIterator i = inner.iterate(); i.hasNext(); ) {			next = i.nextItem();			sum += ((NumericValue)next.convertTo(Type.NUMBER)).getDouble();		}		return new DoubleValue(sum);	}