  Based on the recent developments of explicit computations at 2 loops in
superstring theory in the covariant RNS formalism, we propose an explicit
formula for the arbitrary loop 4-particle amplitude in superstring theory. We
prove that this formula passes two very difficult tests: modular invariance and
factorization. If proved, this shows that superstring theory is not only finite
order by order in perturbation theory but is also exceptionally simple.
