Function assemble_lyrics 

pub fn assemble_lyrics(
    stream: &Stream,
    line_number: IntegerType,
    word_separator: &str,
) -> String

The lyrics of one line of a stream, nested streams included, joined into words, the syllables of each word run together: music21’s assembleLyrics. line_number counts the lyrics on each note from one, as music21 does, so nought reads each note’s last lyric; a note without that many lyrics is passed over, and so is a syllable that is only an underscore, as many scores write a held syllable.