“…For the two-dimensional decomposition conjecture pertinent to sigma models on gerbes, there is now abundant evidence, including all-genera partition function computations in orbifold examples [14], checks in mirror symmetry and quantum cohomology [14], applications to gauged linear sigma models [15], and now checks of predictions for Gromov-Witten invariants [33,34,35,36,37,38]. By contrast, in the four-dimensional case above, we have no independent evidence, no examples, only the arguments above.…”