Format expr as LilyPond input and write to output file name.
Write t to foo.ly in the current directory.
abjad> t = Note(0, (1, 4))
abjad> write_ly(t, 'foo.ly')
Write t to foo.ly in the /home/user directory. Include the paris.ly template in foo.ly.
abjad> t = Note(0, (1, 4))
abjad> write_ly(t, '/home/user/foo.ly', 'paris')
New in version 1.1.1: Optional footer keyword.