phor</a></li>
  <li><a href="#the-model-specification" id="toc-the-model-specification" class="nav-link" data-scroll-target="#the-model-specification">The model specification</a></li>
  <li><a href="#shapes-of-models" id="toc-shapes-of-models" class="nav-link" data-scroll-target="#shapes-of-models">“Shapes” of models</a>
  <ul class="collapse">
  <li><a href="#one-explanatory-variable" id="toc-one-explanatory-variable" class="nav-link" data-scroll-target="#one-explanatory-variable">One explanatory varia