I'm in the middle of writing a conference presentation which talks about some of what Mango does, I'm struggling for an example, and bam! There it is, what I need is an iterator that traverses a list in reverse order. So I wrote a test, wrote an iterator, then wrote it again in under five minutes. It's in subversion now.
You can pull the Mango code from
svn co svn://jezuk.dnsalias.net/jezuk/mango/trunk