my $cacheDir = getSCMCacheDir . "/bzr";
    mkpath($cacheDir);
    my $clonePath = $cacheDir . "/" . sha256_hex($uri);
    if (! -d $clonePath) {
        (my $res, $stdout, $stderr) = captureStdoutStderr(600, "bzr", "branch", $uri, $clonePath);
        die "error cloning bazaar branch at `$uri':\n$stderr" if $res;
    }
    chdir $clonePath or die $!;
    (my $res, $stdout, $stderr) = captureStdoutStderr(600, "bzr", "pull");
    die "error pulling latest change bazaar branch at `$uri':\n$stderr" if $res;

    my @cmd = (["bzr", "revno"], "|", ["sed", 's/^ *\([0-9]*\).*/\1/']);

    my @cmd = (["bzr", "revno", $uri], "|", ["sed", 's/^ *\([0-9]*\).*/\1/']);

            "nix-prefetch-bzr", $clonePath, $revision);

            "nix-prefetch-bzr", $uri, $revision);