  Sensitivity of a neutrino factory to various mixing angles in a scheme with
one sterile neutrino is studied using $\nu_e\to\nu_\mu$, $\nu_\mu\to\nu_\mu$,
$\nu_e\to\nu_\tau$ and $\nu_\mu\to\nu_\tau$. While the "discovery-channel"
$\nu_\mu\to\nu_\tau$ is neither useful in the standard three flavor scheme nor
very powerful in the sensitivity study of sterile neutrino mixings, this
channel is important to check unitarity and to probe the new CP phase in the
scheme beyond the standard neutrino mixing framework.
