public DocumentSet preselect(DocumentSet in_docs, StaticContext context) throws XPathException {
public DocumentSet preselect(DocumentSet in_docs) throws XPathException {
	public DocumentSet preselect(DocumentSet in_docs, StaticContext context) throws XPathException {        //return in_docs;		DocumentSet left_docs = left.preselect(in_docs, context);		DocumentSet right_docs = right.preselect(in_docs, context);		return left_docs.union(right_docs);	}