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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _registerExtension(self, extension, rtiRegItem):
 """ Links an file name extension to a repository tree item. """

        check_is_a_string(extension)
        check_class(rtiRegItem, RtiRegItem)

        logger.debug("  Registering extension {!r} for {}".format(extension, rtiRegItem))

        # TODO: type checking
        if extension in self._extensionMap:
            logger.info("Overriding extension {!r}: old={}, new={}"
                        .format(extension, self._extensionMap[extension], rtiRegItem))
        self._extensionMap[extension] = rtiRegItem