  The decay $\Xi^-_b \to \pi^- \Lambda_b$ has recently been observed by the
LHCb Collaboration at CERN. In contrast to most weak decays of $b$-flavored
baryons, this process involves the decay of the strange quark in $\Xi_b$, and
thus has features in common with nonleptonic weak decays of hyperons. Thanks to
the expected pure S-wave nature of the decay in question in the heavy $b$ quark
limit, we find that its amplitude may be related to those for S-wave
nonleptonic decays of $\Lambda$, $\Sigma$, and $\Xi$ in a picture inspired by
duality. The calculated branching fraction ${\cal B}(\Xi^-_b \to \pi^-
\Lambda_b) = (6.3 \pm 4.2) \times 10^{-3}$ is consistent with the range allowed
in the LHCb analysis. The error is dominated by an assumed 30\% uncertainty in
the amplitude due to possible U(3) violation. A more optimistic view based on
sum rules involving nonleptonic hyperon decay S-wave amplitudes reduces the
error on the branching fraction to $2.0 \times 10^{-3}$.
