prevKey = key;
          }
        }
        return new ImmutableSortedMap<>(
            new RegularImmutableSortedSet<K>(ImmutableList.<K>asImmutableList(keys), comparator),
            ImmutableList.<V>asImmutableList(values));
    }
  }

  /**
   * Returns a builder that creates immutable sorted maps whose keys are ordered by their natural