@Override
  public ImmutableSortedMap<K, V> headMap(K toKey) {
    return headMap(toKey, false);
  }
@Override
  public ImmutableSortedMap<K, V> headMap(K toKey) {
    return headMap(toKey, false);
  }
@ Override public ImmutableSortedMap < K , V > headMap ( K toKey ) { return headMap ( toKey , false ) ; }
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}.
This method returns a { @code ImmutableSortedMap } consisting of the entries whose keys are less than { @code toKey } .