def compile_ly ly_filename
  system "lilypond --svg --output=#{tmp SongsDir} #{ly_filename}"
  system "lilypond --output=#{tmp SongsDir} #{ly_filename}"
end

def make_sheets songs
  songs.keys.each do |song_id|
    ly_filename = src(song_ly song_id)
    if File.file? ly_filename
      compile_ly ly_filename
      system "mv #{tmp (song_tmp_svg song_id)} #{build(song_svg song_id)}"
    end
  end
end