two of the free music typesetting applications most popular among church musicians are LilyPond and Gregorio. lygre gem currently provides tool *grely* converting Gregorio's input format (gabc) to simple LilyPond input. In future another tool for the other direction of conversion may be added. As rb-music-theory is not at RubyGems, either install it before attempting to 'gem install lygre', or get lygre's source from github and install the dependencies by 'bundle install'.