n use for the called function.
    </p>
<p>
      The default version of this function returns zero. 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_error</span></code> C++ exception.
    </p>
<h4>
<a name="math_toolkit.error_handling.h7"></a>
      <span class="phrase"><a name="math_toolkit.e