private void appendGradientDefs(Document document, Element defs, Set<String> colors) {
        // default
        if(!colors.contains(defaultColor)) {
            defs.appendChild(createGradientForNode(document, defaultColor));
        }
        for(String color : colors) {
            if(!color.equals("none"))
                defs.appendChild(createGradientForNode(document, color));
        }
    }