my $uri = $value;

    # Allow users to specify a revision number next to the URI.
    my ($uri, $revision) = split ' ', $value;

    # First figure out the last-modified revision of the URI.
    my @cmd = (["svn", "ls", "-v", "--depth", "empty", $uri],
               "|", ["sed", 's/^ *\([0-9]*\).*/\1/']);

    die "Cannot get head revision of Subversion repository at `$uri':\n$stderr"
        unless IPC::Run::run(@cmd, \$stdout, \$stderr);
    my $revision = $stdout; chomp $revision;

    unless (defined $revision) {
	# First figure out the last-modified revision of the URI.
	my @cmd = (["svn", "ls", "-v", "--depth", "empty", $uri],
		   "|", ["sed", 's/^ *\([0-9]*\).*/\1/']);
	die "Cannot get head revision of Subversion repository at `$uri':\n$stderr"
	    unless IPC::Run::run(@cmd, \$stdout, \$stderr);
	$revision = $stdout; chomp $revision;
    }