def search(self, regex):
        try:
            spec = {'$or': [{'data.content'    : {'$regex': regex, '$options': 'im'}},
                            {'data.description': {'$regex': regex, '$options': 'im'}}]}
            fields = {'_id': 0}
            cur = self.db.docs.find(spec, fields)
        except Exception as e:
            raise DatabaseException(e)
        return [Document(**row) for row in cur]
def kth(root, k):
    if k < size(root.left):
        return kth(root.left, k)
    elif k > size(root.left):
        return kth(root.right, k - size(root.left) - 1)
    return root.key