\define href()
<$macrocall $name="datauri" title="image.png" $output="text/plain"/>
\end
\define Fancybox()
<a href=<<href>> >
macro test
</a>
\end
<<Fancybox>>
but this doesn’t seem to work, I got this
<a href="<$macrocall $name="datauri" title="image.png" $output="text/plain"/>">
macro test
</a>