exclusive_union_of xunion_of worm_anatomy T1 xunion_of T2 means that T1 is the union of T2 and one (or more) other term, exclusively. Thus, given T1 xunion_of T2 and T1 xunion_T3, one can infer that any instance of T1 must be an instance of T2 or T3 but not T2 and T3.