code: @Nonnull
      public <B> HashTable<K, B> toHashTable(@Nonnull F2<K, ImmutableList<V>, B> conversion) {
          return this.data.foldLeft((acc, p) -> acc.put(p.left, conversion.apply(p.left, p.right)), HashTable.empty(this.data.hasher));
      }
docs: version: key is used