code: @Override
    public ImmutableSortedMap<K, V> headMap(K toKey, boolean inclusive) {
      return getSubMap(0, keySet.headIndex(checkNotNull(toKey), inclusive));
    }
docs: This method returns a {@code ImmutableSortedMap}, consisting of the entries whose keys are less
  than (or equal to, if {@code inclusive}) {@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}.
  
  @since 12.0