src/main/java/org/jfree/graphics2d/svg/SVGGraphics2D.java
private String registerClip(Shape clip) {
        if (clip == null) {
            this.clipRef = null;
            return null;
        }
        // generate the path
        String pathStr = getSVGPathData(new Path2D.Double(clip));
        int index = this.clipPaths.indexOf(pathStr);
        if (index < 0) {
            this.clipPaths.add(pathStr);
            index = this.clipPaths.size() - 1;
        }
        return this.defsKeyPrefix + CLIP_KEY_PREFIX + index;
    }
private String registerClip(Shape clip) {
        if (clip == null) {
            this.clipRef = null;
            return null;
        }
        // generate the path
        String pathStr = getSVGPathData(new Path2D.Double(clip));
        int index = this.clipPaths.indexOf(pathStr);
        if (index < 0) {
            this.clipPaths.add(pathStr);
            index = this.clipPaths.size() - 1;
        }
        return this.defsKeyPrefix + CLIP_KEY_PREFIX + index;
    }
Registers the clip so that we can later write out all the clip
definitions in the DEFS element.

@param clip  the clip (ignored if {@code null})