  Inverse muon decay, $\nu_\mu e^-\to\mu^-\nu_e$, is a reaction whose
cross-section can be predicted with very small uncertainties. It has a neutrino
energy threshold of $\approx 11$ GeV and can be used to constrain the
high-energy part of the flux in the NuMI neutrino beam. This reaction is the
dominant source of events which only contain high-energy muons nearly parallel
to the direction of the neutrino beam. We have isolated a sample of hundreds of
such events in neutrino and anti-neutrino enhanced beams, and have constrained
the predicted high-energy flux.
