of the falling factorial of x with power n defined for
positive x and real n. <span class="math display">\[ \mathrm{log\_falling\_factorial}(x,n) =
\begin{cases} \log (x)_n &amp; \text{if } x &gt; 0 \\ \textrm{error} &amp;
\text{if } x \leq 0 \end{cases} \]</span></p>
<!-- real; rising_factorial; (real x, real n); -->
<p></p>
<p><code>real</code> <strong><code>rising_factorial</code></strong><code>(real x, real n)</code><br>
Return the rising factorial of x with power n defined for positive x
and real n. 