code: public static <T extends Comparable<? super T>> Collector<T, ?, OptionalLong> minIndex() {
          return minIndex(Comparator.naturalOrder());
      }
docs: Returns a {@code Collector} which finds the index of the minimal stream
  element according to the elements natural order. If there are several
  minimal elements, the index of the first one is returned.
  
  @param <T> the type of the input elements
  @return a {@code Collector} which finds the index of the minimal element.
  @see #minIndex(Comparator)
  @since 0.3.5