private void generateHeader(PrintStream out) {
        out.println("digraph G {");
        out.println("    ranksep = " + repository.meta("rankSep", ""+rankSep));
        out.println("    rankdir = " + repository.meta("direction", ""+direction));
        if(repository.hasMeta("nodesep"))
        out.println("    nodesep = " + repository.meta("nodesep"));
    }