  The complete proof of cutting rules needed for proving perturbative unitarity
of quantum field theories usually employs the largest time equation or old
fashioned perturbation theory. None of these can be generalized to string field
theory that has non-local vertices. In arXiv:1604.01783 we gave a proof of
cutting rules in string field theory, which also provides an alternative proof
of cutting rules in ordinary quantum field theories. In this note we illustrate
how this works for the box diagram of $\phi^4$ field theory, avoiding the
contributions from anomalous thresholds.
