http://www.umiacs.umd.edu/~resnik/ling773_sp2011/readings/dyer-inside-outside.pdf https://www.cs.jhu.edu/~jason/465/iobasics.pdf http://curtis.ml.cmu.edu/w/courses/index.php/Inside_Outside_algorithm http://vcla.stat.ucla.edu/sig12/file/Lect_4_Todorovic_Inside_Outside_Algorithm.pdf https://en.wikipedia.org/wiki/Inside%E2%80%93outside_algorithm