code: private void appendOptionalElementIDFromHint(StringBuilder sb) {
          String elementID = (String) this.hints.get(SVGHints.KEY_ELEMENT_ID);
          if (elementID != null) {
              this.hints.put(SVGHints.KEY_ELEMENT_ID, null); // clear it
              if (this.elementIDs.contains(elementID)) {
                  throw new IllegalStateException("The element id " 
                          + elementID + " is already used.");
              } else {
                  this.elementIDs.add(elementID);
              }
              this.sb.append("id=\"").append(elementID).append("\" ");
          }
      }
docs: A utility method that appends an optional element id if one is
  specified via the rendering hints.
  
  @param sb  the string builder ({@code null} not permitted).