In Ex:ProcedureAdjacency back in Using Variables as Attributes, we found that this attempt to use a simple variable to compose a Wikipedia link didn't work:

<$let wikipedia="https://en.wikipedia.org/wiki/">
  <<wikipedia>>Aardvark
</$let>

Fix this example by replacing the <<wikipedia>>Aardvark line with an a element using a {{{ filtered transclusion }}}.

Tip: The filter operator addsuffix will come in handy.