  In the twisted M-theory setting, various types of fusion of M2 and M5 branes
induce coproducts between the algebra of operators on M2 branes and the algebra
of operators on M5 branes. By doing a perturbative computation in the gravity
side, which is captured by the 5d topological holomorphic $U(1)$ Chern-Simons
theory, we reproduce the non-perturbative coproducts.
