the tree being fetched from a Subversion repository. The revision
number is now optional (and defaults to HEAD).
This makes `fetchsvn' more pure. First, a URL/revision tuple does
not uniquely identify a file resource, since the repository itself
might change. Second, `svn:external' attributes can cause arbitrary
resources to be exported.
A script `nix-prefetch-svn' has been provided to determine the hash
of a URL.
svn path=/nixpkgs/trunk/; revision=938