Time signature with alternate in parentheses

In polymetric music, a composer may indicate beat structure by following one time signature with another in parenthesis, for example, ‘3/4 (6/8)’ indicating a 6/8 beat in a 3/4 measure, or ‘6/4 (6/8)’ indicating a 6/8 beat in a 6/4 measure.

This snippet provides a music function \timeP that is a small Scheme wrapper around the \compound-meter markup function. It overrides the TimeSignature stencil once to display both a regular and an alternate time signature.

\version "2.24"

timeP =
#(define-music-function (t tp) (list? list?)
   (define ((time-parenthesized-time t tp) grob)
     (let ((m1 (grob-interpret-markup
                grob (markup #:compound-meter t)))
           (m2 (grob-interpret-markup
                grob (markup #:compound-meter tp))))
       (ly:stencil-combine-at-edge
        m1 X RIGHT
        (parenthesize-stencil m2 0.1 0.4 0.4 0.1)
        0.3)))
   #{
     \once \override Timing.TimeSignature.stencil =
       #(time-parenthesized-time t tp)
   #})

\relative c' {
  \timeP #'(3 4) #'(6 8) \time 3/4
    b8 b8 gis8 gis4 gis8 |
  \timeP #'(4 4) #'(3 2 3 8) \time 4/4
    b4. b4 b4.
}