OK, WILCO.

About this (for my own records):

<i>I'm wondering why font-18.1 does not fail though. [b2677cc770] is a hint that is should fail identically.</i>

This is because in this test we have <pre>set x [join {Courier 12} { }]</pre>, i.e. join receives two items, and the optimization made in


does therefore not enter in play.