code: @Beta
    protected SortedMap<K, V> standardSubMap(K fromKey, K toKey) {
      checkArgument(unsafeCompare(fromKey, toKey) <= 0, "fromKey must be <= toKey");
      return tailMap(fromKey).headMap(toKey);
    }
docs: A sensible default implementation of {@link #subMap(Object, Object)} in terms of {@link
  #headMap(Object)} and {@link #tailMap(Object)}. In some situations, you may wish to override
  {@link #subMap(Object, Object)} to forward to this implementation.
  
  @since 7.0