Slur with two turning points

A slur normally can only have one turning point. In special situations, one may need two or more turning points, however. This can be achieved by linking two (or more) slur stencils together. The control points need to be specified manually: for two segments this is a set of seven points, of which the fourth is shared by both segments. To ease working with the function, the control points are shown as light blue crosses if the second argument is ##t.
For a more refined solution to the problem, see Shaping Bezier Curves, needing version 2.19.48 or higher.

\version "2.24.0"

% put together by Harm and Simon Albrecht
% in <http://lists.gnu.org/archive/html/lilypond-user/2016-09/msg00441.html> ff.

#(define (make-cross-stencil coord)
   "Draw a cross-stencil at coord."
   (let ((thick 0.1)
         (sz 0.2))
     (stencil-with-color
      (ly:stencil-add
       (make-line-stencil
        thick
        (- (car coord) sz)
        (- (cdr coord) sz)
        (+ (car coord) sz)
        (+ (cdr coord) sz))
       (make-line-stencil
        thick
        (- (car coord) sz)
        (+ (cdr coord) sz)
        (+ (car coord) sz)
        (- (cdr coord) sz)))

      cyan)
     ))

compoundSlur =
#(define-event-function (contr-pts ann?) (list? boolean?)
   (let ((proc (lambda (grob)
                 (let*  (;; only here for reference:
                          (cps (ly:slur::calc-control-points grob))
                          (cps1 (list
                                 (first contr-pts)
                                 (second contr-pts)
                                 (third contr-pts)
                                 (fourth contr-pts)))
                          (cps2 (list
                                 (fourth contr-pts)
                                 (fifth contr-pts)
                                 (sixth contr-pts)
                                 (seventh contr-pts)))
                          (first-spline-stil
                           (begin
                            (ly:grob-set-property! grob 'control-points cps1)
                            (ly:slur::print grob)))
                          (second-spline-stil
                           (begin
                            (ly:grob-set-property! grob 'control-points cps2)
                            (ly:slur::print grob)))
                          ;; annotates all new control-points
                          (crosses
                           (if ann?
                               (apply
                                ly:stencil-add
                                (map
                                 (lambda (c)
                                   (make-cross-stencil c))
                                 (append cps1 cps2)))
                               empty-stencil))
                          )
                   ;(pretty-print cps)

                   (ly:stencil-add
                    first-spline-stil
                    second-spline-stil
                    crosses)))))
     #{ -\tweak stencil $proc ( #}))

cptsA = #'((0.6 . 5.8)
           (3.6 . 7.8)
           (25.0 . 5.0)
           (32 . 12)
           (39.0 . 17.5)
           (53.1 . 16.0)
           (55.5 . 12.5))


upper = \relative {
  \key d \major
  \clef bass
  s2 r8 d,16 g b d g b
  d8 r s2.
  s4 \voiceTwo b8.(-- c16-- b2--)
}
lower = \relative {
  \key d \major
  \clef bass
  r2
  <d' b g=>~-^-\compoundSlur \cptsA ##t
  <<
    {
      <d b g>4 <c g e> <b g> \voiceOne cis
      \change Staff = upper
      \clef treble \voiceOne d e fis2)
      \fermata
    }
    \new Voice {
      \voiceTwo
      s2. <g,= e>4 \oneVoice
      <fis b,> <g e> <fis dis>2\fermata
    }
  >>
}
\score {
  <<
    \new PianoStaff <<
      \new Staff = upper \upper
      \new Staff = lower \lower
    >>
  >>
}