joiner and map, strange behavior

[bearophile]

H. S. Teoh:

> I was the one responsible for the recent rewrite of joiner; 
> I'll think
> over how to fix it so that .front is only evaluated once.

This is a strictly related issue, about filter:
http://d.puremagic.com/issues/show_bug.cgi?id=9674

Bye,
bearophile