def build(cls, shape_y, shape_z, sids, initvalue=0., dtype=F64):
        """
        :param shape_y: the total number of intensity measure levels
        :param shape_z: the number of inner levels
        :param sids: a set of site indices
        :param initvalue: the initial value of the probability (default 0)
        :returns: a ProbabilityMap dictionary
        """
        dic = cls(shape_y, shape_z)
        for sid in sids:
            dic.setdefault(sid, initvalue, dtype)
        return dic