code: public static NestableFieldValidator listFv(Class cls, boolean nullAllowed) {
          return listFv(fv(cls, false), nullAllowed);
      }
docs: Returns a new NestableFieldValidator for a List of the given Class.
  
  @param cls         the Class of elements composing the list
  @param nullAllowed whether or not a value of null is valid
  @return a NestableFieldValidator for a list of the given class