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

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)