def get_data_length(self):
if not self.has_data_length():<EOL><INDENT>raise IllegalState()<EOL><DEDENT>else:  <EOL><INDENT>raise Unimplemented()<EOL><DEDENT>
Gets the length of the data represented by this content in
        bytes.

        return: (cardinal) - the length of the data stream
        raise:  IllegalState - has_data_length() is false
        compliance: mandatory - This method must be implemented.