code: public static <T extends Iterable<?>> T checkContentsNotNull(T iterable) {
      if (containsOrIsNull(iterable)) {
        throw new NullPointerException();
      }
      return iterable;
    }
docs: Ensures that an {@code Iterable} object passed as a parameter to the
  calling method is not null and contains no null elements.
  
  @param iterable the iterable to check the contents of
  @return the non-null {@code iterable} reference just validated
  @throws NullPointerException if {@code iterable} is null or contains at
  least one null element