Using tags to produce mensural and modern music from the same source

Using tags it is possible to produce both mensural and modern notation from the same music. In this snippet, a function \menrest is introduced, allowing mensural rests to be pitched as in the original, but with modern rests in the standard staff position.

Tags can also be used where other differences are needed: for example using “whole measure rests” (R1, R\breve, etc.) in modern music, but normal rests (r1, r\breve, etc.) in the mensural version. Converting mensural music to its modern equivalent is usually referred to as transcription.

The call c4.\Be c8 c\Am is the same as c4.[ c8 c]. However, it suppresses warnings if it starts on a note that can't hold a beam but needs it anyway due to the use of Completion_heads_engraver.

[Note that the custos sticks out into the right margin and might be cut off if the LilyPond output gets cropped, as is done in both this Wiki and the official documentation. The use of \with-true-dimensions avoids this.]

\version "2.24"

\layout {
  line-width = 150\mm
}

menrest = #(define-music-function (note) (ly:music?)
              #{
                \tag #'mens $(make-music 'RestEvent note)
                \tag #'mod $(make-music 'RestEvent note 'pitch '())
              #})
Be = \tag #'mod
       #(begin
          (ly:expect-warning (G_ "stem does not fit in beam"))
          (ly:expect-warning (G_ "beam was started here"))
          (make-span-event 'BeamEvent START))
Am = \tag #'mod ]

MenStyle = {
  \override Score.BarNumber.transparent = ##t
  \override Stem.neutral-direction = #up
  \omit Slur
  \omit Beam
}

finalis = \section

Music = \relative c'' {
  \key f \major
  g1 d'2 \menrest bes4 bes a2 \menrest r4 g4 fis4.
  fis8 fis4 fis g e f4.([ g8] a4[ g8 f]
    g2.\Be fis8 e\Am fis2) g\breve \finalis
}

MenLyr = \lyricmode {
  So farre, deere life, deare life,
  from thy bright beames ab- ſen- ted,
}
ModLyr = \lyricmode {
  So far, dear life, dear life,
  from your bright beams ab -- sen -- ted, __
}

\markup \with-true-dimensions % work around a cropping issue
\score {
  \keepWithTag #'mens {
    <<
      \new PetrucciStaff {
        \new PetrucciVoice = "Cantus" {
          \clef "petrucci-c1" \time 4/4 \MenStyle \Music
        }
      }
      \new Lyrics \lyricsto "Cantus" \MenLyr
    >>
  }
  \layout {
    \context {
      \PetrucciVoice
      % No longer necessary starting with version 2.25.23.
      \override Flag.style = #'mensural
    }
  }
}

\markup\vspace #1

\score {
  \keepWithTag #'mod {
    \new ChoirStaff <<
      \new Staff {
        \new Voice = "Sop" \with {
          \remove "Note_heads_engraver"
          \consists "Completion_heads_engraver"
          \remove "Rest_engraver"
          \consists "Completion_rest_engraver"
        } \shiftDurations 1 0 { \time 2/4 \autoBeamOff \Music }
      }
      \new Lyrics \lyricsto "Sop" \ModLyr
    >>
  }
}