code: public static <T extends Describable<T>>
      List<T> newInstancesFromHeteroList(StaplerRequest req, JSONObject formData, String key,
                  Collection<? extends Descriptor<T>> descriptors) throws FormException {
  
          return newInstancesFromHeteroList(req,formData.get(key),descriptors);
      }
docs: Used to build {@link Describable} instance list from {@code <f:hetero-list>} tag.
  
  @param req
  Request that represents the form submission.
  @param formData
  Structured form data that represents the contains data for the list of describables.
  @param key
  The JSON property name for 'formData' that represents the data for the list of describables.
  @param descriptors
  List of descriptors to create instances from.
  @return
  Can be empty but never null.