nt <code>context$X</code> (d x k context matrix),
<code>context$k</code> (number of arms) and <code>context$d</code> (number of context features)
(as set by <code>bandit</code>).</p></li>
<li><p><code>action</code>:  list, containing <code>action$choice</code> (as set by <code>policy</code>).</p></li>
</ul>
returns a named <code>list</code> containing <code>reward$reward</code> and, where computable,
<code>reward$optimal</code> (used by "oracle" policies and to calculate regret).</p></dd>
    <dt><code><a h