code: public SortedSet<ClassDoc> implementingclasses(ClassDoc cd) {
          SortedSet<ClassDoc> result = get(implementingclasses, cd);
          SortedSet<ClassDoc> intfcs = allSubs(cd, false);
  
          //If class x implements a subinterface of cd, then it follows
          //that class x implements cd.
          Iterator<ClassDoc> subInterfacesIter = intfcs.iterator();
          while (subInterfacesIter.hasNext()) {
              Iterator<ClassDoc> implementingClassesIter
                      = implementingclasses(subInterfacesIter.next()).iterator();
              while (implementingClassesIter.hasNext()) {
                  ClassDoc c = implementingClassesIter.next();
                  if (!result.contains(c)) {
                      result.add(c);
                  }
              }
          }
          return result;
      }
docs: Return the list of classes which implement the interface passed.
  
  @param cd interface whose implementing-classes list is required.