diff --git a/impl-in-typst.html b/impl-in-typst.html
index 7671c45..8ca4d1a 100644
--- a/impl-in-typst.html
+++ b/impl-in-typst.html
@@ -191,7 +191,7 @@ <h2 id="vertical-bar-for-evaluation">A vertical bar for evaluation</h2>
 <br/>
 <p>
   Also it's common to use a vertical bar to denote the deference of function values at two points, especially when calculating definite integrals.
-  e.g.
+  e.g.<sup><a href="#ref-evaluation">[3]</a></sup>
 </p>
 <div class="render-m">F(x) \bigg\rvert_{a}^{b}</div>