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 uses a small Scheme function that overrides the TimeSignature stencil once to display both the regular and the alternate time signature.

\version "2.24"

#(define ((time-parenthesized-time up down upp downp) grob)
   (let ((m1 (grob-interpret-markup
              grob
              (markup #:override '(baseline-skip . 0)
                      #:number (#:center-column
                                ((number->string up)
                                 (number->string down))))))
         (m2 (grob-interpret-markup
              grob
              (markup #:override '(baseline-skip . 0)
                      #:number (#:center-column
                                ((number->string upp)
                                 (number->string downp)))))))
     (ly:stencil-combine-at-edge
      m1 X RIGHT
      (parenthesize-stencil m2 0.1 0.4 0.4 0.1)
      0.3)))

\relative c' {
  \once \override Staff.TimeSignature.stencil =
    #(time-parenthesized-time 3 4 6 8)
  \time 3/4 b8 b8 gis8 gis4 gis8 |
  \time 4/4 b4 b b b
}