BlockTree terms dictionary.

This terms dictionary organizes all terms into blocks according to shared prefix, such that each block has enough terms, and then stores the prefix trie in memory as an FST as the index structure. It allows you to plug in your own {@link org.apache.lucene.codecs.PostingsBaseFormat} to implement the postings.

See {@link org.apache.lucene.codecs.blocktree.BlockTreeTermsWriter} for the file format.