'V.dominos_tuple(X) -> tuple of idsets
Return a tuple of dominos for the tuple of sets of objects X.'

def dominos_tuple(self, X):

    D_ = [self.nodeset_adapt(x) for x in X]
    T = self.hv.reachable
    S = self.immnodeset([self.root])
    D = self.immnodeset_union(D_)
    W = T(S, D)
    return tuple([self.retset((T(Di, W) - T(D, (W | Di)))) for Di in D_])
