docs: def add_speaker(self, collection_name, metadata):
          """Add a new speaker to this collection.
  
          :type collection_name: String
          :param collection_name: the name of the collection to search
          :type metadata: Dict
          :param metadata: dictionary of metadata properties and values
            for this speaker. Must include 'dcterms:identifier' a unique
            identifier for the speaker.
  
          :rtype: String
          :returns: the URL of the newly created speaker, or None if there was an
              error
          """
  
          if 'dcterms:identifier' not in metadata:
              raise APIError(msg="No identifier in speaker metadata")
  
          if '@context' not in metadata:
              metadata['@context'] = CONTEXT
  
          speakers_url = "/speakers/"+collection_name+"/"
          resp = self.api_request(speakers_url, data=json.dumps(metadata), method="POST")
  
          if 'success' in resp:
              return resp['success']['URI']
          else:
              return None