How to downgrade to a previous version of libxslt?

[Lyle Johnson]

Apologies if this is a FAQ.

I'd like to downgrade my libxslt installation from 1.1.20 (the current
version) to 1.1.15. I've deactivated 1.1.20, but when I try to install
1.1.15:

    sudo port install libxslt at 1.1.15

I can see that it's trying to fetch libxslt-1.1.20.tar.gz from the
source web site.