  The recently-observed decay $B^0 \to D_s^+ \pi^-$ is expected to proceed
mainly by means of a tree amplitude in the factorization limit: $B^0 \to \pi^-
{(W^+)}^*$, ${(W^+)}^* \to D_s^+$. Under this assumption, we predict the
corresponding contribution of the tree amplitude to $B^0 \to \pi^+ \pi^-$. We
indicate the needed improvements in data that will allow a useful estimate of
this amplitude with errors comparable to those accompanying other methods.
Since the factorization hypothesis for this process goes beyond that proved in
most approaches, we also discuss independent tests of this hypothesis.
