def open(self, url):
        """
        Open a document at the specified url.
        @param url: A document URL.
        @type url: str
        @return: A file pointer to the document.
        @rtype: StringIO
        """
        protocol, location = self.split(url)
        if protocol == self.protocol:
            return self.find(location)
        else:
            return None
Open a document at the specified url.
        @param url: A document URL.
        @type url: str
        @return: A file pointer to the document.
        @rtype: StringIO