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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 get_content_descendants_by_type(self, content_id, child_type, expand=None, start=None, limit=None, callback=None):
 """ Returns the direct descendants of a piece of Content, limited to a single descendant type. The {@link ContentType}(s) of the descendants returned is specified by the "type" path parameter in the request. Currently the only supported descendants are comment descendants of non-comment Content. :param content_id (string):
 A string containing the id of the content to retrieve descendants for :param child_type (string):
 A {@link ContentType} to filter descendants on. :param expand (string):
 OPTIONAL: A comma separated list of properties to expand on the descendants. Default: Empty :param start (int):
 OPTIONAL: The index of the first item within the result set that should be returned. Default: 0. :param limit (int):
 OPTIONAL: How many items should be returned after the start index. Default: 25 or site limit. :param callback: OPTIONAL: The callback to execute on the resulting data, before the method returns. Default: None (no callback, raw data returned). :return: The JSON data returned from the content/{id}/descendant/{type} endpoint, or the results of the callback. Will raise requests.HTTPError on bad input, potentially. """

        params = {}
        if expand:
            params["expand"] = expand
        if start is not None:
            params["start"] = int(start)
        if limit is not None:
            params["limit"] = int(limit)
        return self._service_get_request("rest/api/content/{id}/descendant/{type}"
                                         "".format(id=content_id, type=child_type), params=params, callback=callback)