code: public static ClassDoc getFirstVisibleSuperClassCD(ClassDoc classDoc,
              Configuration configuration) {
          if (classDoc == null) {
              return null;
          }
          ClassDoc supClassDoc = classDoc.superclass();
          while (supClassDoc != null &&
                    (! (supClassDoc.isPublic() ||
                                isLinkable(supClassDoc, configuration))) ) {
              supClassDoc = supClassDoc.superclass();
          }
          if (classDoc.equals(supClassDoc)) {
              return null;
          }
          return supClassDoc;
      }
docs: Given a class, return the closest visible super class.
  
  @param classDoc the class we are searching the parent for.
  @param configuration the current configuration of the doclet.
  @return the closest visible super class.  Return null if it cannot
  be found (i.e. classDoc is java.lang.Object).