code: public Trie2_16 toTrie2_16() {
          Trie2_16 frozenTrie = new Trie2_16();
          freeze(frozenTrie, ValueWidth.BITS_16);
          return frozenTrie;
      }
docs: Produce an optimized, read-only Trie2_16 from this writable Trie.
  The data values outside of the range that will fit in a 16 bit
  unsigned value will be truncated.