Merge two regions into one.

        The merged region will take on the id1 identifier.

        :param id1: region 1 identifier
        :param id2: region 2 identifier

def merge_regions(self, id1, id2):
        """Merge two regions into one.

        The merged region will take on the id1 identifier.

        :param id1: region 1 identifier
        :param id2: region 2 identifier
        """
        region2 = self.region_by_identifier(id2)
        self[region2] = id1