code><br>
Return the natural logarithm of the difference of the natural
exponentiation of x and the natural exponentiation of y. <span class="math display">\[
\mathrm{log\_diff\_exp}(x,y) = \begin{cases} \log(\exp(x)-\exp(y)) &amp;
\text{if } x &gt; y \\[6pt] \textrm{NaN} &amp; \text{otherwise} \end{cases}
\]</span></p>
<!-- real; log_mix; (real theta, real lp1, real lp2); -->
<p></p>
<p><code>real</code> <strong><code>log_mix</code></strong><code>(real theta, real lp1, real lp2)</code><br>
Return the log m