docs: def _addProteinIdsToGroupMapping(self, proteinIds, groupId):
          """Add a groupId to one or multiple entries of the internal
          proteinToGroupId mapping.
  
          :param proteinIds: a proteinId or a list of proteinIds, a proteinId
              must be a string.
          :param groupId: str, a groupId
          """
          for proteinId in AUX.toList(proteinIds):
              self._proteinToGroupIds[proteinId].add(groupId)