public Builder() {}

    /** Creates a new builder with a hint for the number of distinct keys. */
    Builder(int expectedKeys) {
      if (expectedKeys > 0) {
        builderMap = Platform.preservesInsertionOrderOnPutsMapWithExpectedSize(expectedKeys);
      }
      // otherwise, leave it null to be constructed lazily
    }

    Map<K, ImmutableCollection.Builder<V>> ensureBuilderMapNonNull() {