TODO
  * grammar
  * parser
  * other semirings
  * sparse vector (unordered_map)
  * hg serialization? json/bson/msgpack/protocol buffers (no!)
  * hg: json input (jsoncpp?)
  * language model: kenlm

depends on msgpack [1]
http://jscheiny.github.io/Streams/

[1] http://msgpack.org