}
    public void visitNode(FlatNode node)     {        if (node.isFilter()) {            SIRFilter fnode = (SIRFilter)node.contents;            if (debugging) {            System.err.println(this.getClass().getName() + " processing filter "                     + fnode.getName() + ": "                     +  fnode.getInputType() + "->" + fnode.getOutputType());            passedTypes.add(fnode.getOutputType());            passedTypes.add(fnode.getInputType());            }        }    }