&#160;18.&#160;Policies: Controlling Precision, Error Handling etc">Policy</a>
      is the current policy in use for the called function.
    </p>
<p>
      The default version of this function returns <span class="emphasis"><em>z</em></span>. But with
      another policy, like <code class="computeroutput"><span class="identifier">throw_on_error</span></code>
      throws an <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">underflow_er