li><a href="#stan-functions-4" id="toc-stan-functions-4" class="nav-link" data-scroll-target="#stan-functions-4">Stan functions</a></li>
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  <li><a href="#gamma-distribution" id="toc-gamma-distribution" class="nav-link" data-scroll-target="#gamma-distribution">Gamma distribution</a>
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  <li><a href="#probability-density-function-5" id="toc-probability-density-function-5" class="nav-link" data-scroll-target="#probability-density-function-5">Probability density function</a></l