(forest extension) How should one create forest

[Marcin Kasperski]

I decided to take a look at forest extension and I am somewhat 
confused. How should I ***create*** new forest?

Let's say I have three (normal) mercurial repositories
   /some/dir/repo1
   /some/dir/repo2
   /some/dir/repo3
which command should I issue to gather them into a forest?
If I try to run 'hg fsnap' in /some/dir, then it just complains 
that there is no mercurial repo there. Should I create extra
repo in /some/dir?