checkState(this.valueComparator == null, "valueComparator was already set");
      this.valueComparator = checkNotNull(valueComparator, "valueComparator");
      return this;
    }

    @CanIgnoreReturnValue
    Builder<K, V> combine(Builder<K, V> other) {
      checkNotNull(other);
      ensureCapacity(this.size + other.size);
      System.arraycopy(
          other.alternatingKeysAndValues,