docs: def addComment(self, msg=None):
          """Add a comment to this workitem
  
          :param msg: comment message
          :return: the :class:`rtcclient.models.Comment` object
          :rtype: rtcclient.models.Comment
          """
  
          origin_comment = '''
  <rdf:RDF
      xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
      xmlns:rtc_ext="http://jazz.net/xmlns/prod/jazz/rtc/ext/1.0/"
      xmlns:rtc_cm="http://jazz.net/xmlns/prod/jazz/rtc/cm/1.0/"
      xmlns:oslc_cm="http://open-services.net/ns/cm#"
      xmlns:dcterms="http://purl.org/dc/terms/"
      xmlns:oslc_cmx="http://open-services.net/ns/cm-x#"
      xmlns:oslc="http://open-services.net/ns/core#">
    <rdf:Description rdf:about="{0}">
      <rdf:type rdf:resource="http://open-services.net/ns/core#Comment"/>
      <dcterms:description rdf:parseType="Literal">{1}</dcterms:description>
    </rdf:Description>
  </rdf:RDF>
  '''
  
          comments_url = "/".join([self.url,
                                   "rtc_cm:comments"])
          headers = copy.deepcopy(self.rtc_obj.headers)
          resp = self.get(comments_url,
                          verify=False,
                          proxies=self.rtc_obj.proxies,
                          headers=headers)
  
          raw_data = xmltodict.parse(resp.content)
  
          total_cnt = raw_data["oslc_cm:Collection"]["@oslc_cm:totalCount"]
          comment_url = "/".join([comments_url,
                                  total_cnt])
  
          comment_msg = origin_comment.format(comment_url, msg)
  
          headers["Content-Type"] = self.OSLC_CR_RDF
          headers["Accept"] = self.OSLC_CR_RDF
          headers["OSLC-Core-Version"] = "2.0"
          headers["If-Match"] = resp.headers.get("etag")
          req_url = "/".join([comments_url,
                              "oslc:comment"])
  
          resp = self.post(req_url,
                           verify=False,
                           headers=headers,
                           proxies=self.rtc_obj.proxies,
                           data=comment_msg)
          self.log.info("Successfully add comment: [%s] for <Workitem %s>",
                        msg, self)
  
          raw_data = xmltodict.parse(resp.content)
          return Comment(comment_url,
                         self.rtc_obj,
                         raw_data=raw_data["rdf:RDF"]["rdf:Description"])