code: @SuppressWarnings("unchecked")
    public static <K extends Comparable<? super K>, V> ImmutableSortedMap<K, V> of(
        K k1, V v1, K k2, V v2) {
      return ofEntries(entryOf(k1, v1), entryOf(k2, v2));
    }
docs: Returns an immutable sorted map containing the given entries, sorted by the natural ordering of
  their keys.
  
  @throws IllegalArgumentException if the two keys are equal according to their natural ordering