code: @Override
    public ImmutableSortedMap<K, V> headMap(K toKey) {
      return headMap(toKey, false);
    }
docs: This method returns a {@code ImmutableSortedMap}, consisting of the entries whose keys are less
  than {@code toKey}.
  
  <p>The {@link SortedMap#headMap} documentation states that a submap of a submap throws an
  {@link IllegalArgumentException} if passed a {@code toKey} greater than an earlier {@code
  toKey}. However, this method doesn't throw an exception in that situation, but instead keeps
  the original {@code toKey}.