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