code: private String getClipPathRef() {
          if (this.clip == null) {
              return "";
          }
          if (this.clipRef == null) {
              this.clipRef = registerClip(getClip());
          }
          StringBuilder b = new StringBuilder();
          b.append("clip-path=\"url(#").append(this.clipRef).append(")\"");
          return b.toString();
      }
docs: Returns a clip path reference for the current user clip.  This is
  written out on all SVG elements that draw or fill shapes or text.
  
  @return A clip path reference.