Returns the next document with the specified tag. Empty string is no doc is found.

def next_doc_with_tag(self, doc_tag):
        """
        Returns the next document with the specified tag. Empty string is no doc is found.
        """
        while True:
            try:
                doc = next(self)
                if doc.tag == doc_tag:
                    return doc

            except StopIteration:
                raise