Slash Chord Support in Chord Symbol Notation

I propose to extend the chord symbol notation to enable slash chords (also beyond normal inversions).

This may further facilitate the (already phantastic options for) composition with OM in harmonic rich styles like Jazz.

My notation proposal see in option 3.

Musical example (hmmm OK, maybe a bit too simple as the bass is in the the chord, but still hopefully useful to illustrate the use-case):

;; Proposal for slash chord symbol notation support in OM
;;
;; Slash chords: https://en.wikipedia.org/wiki/Slash_chord
;;  - so these include inversions (as implemented already in OM)
;;  - BUT also other bass pitches not being present in original chord
;;  - Latter require special handling and are not supported in OM yet 
;;    (in chord symbols, as far as I know).
;;
;; Musical example: Stairway to Heaven (Led Zeppelin, Jimmy Page)
;; Chords:
;; Am, E/G#, C/G, D/F#, Fmaj7
;;


;; Option 1 -------------------
;
; This traditional way works somewhat, but it is a bit unnatural
; Cons: 
;      - rh and lh are separated and therefore harder to reason about and compose
;      - slash chords are not shown fully with bass as symbols in score

(setf stairway-rh '(w (a4 m)
                      (e4 maj)
                      (c4 maj)
                      (d4 maj)
                      (f4 maj7)))
(setf stairway-lh '(w a2 gs2 g2 fs2 f2))
(ps 'gm
    :title "Stairway to Heaven (Led Zeppelin, Jimmy Page) - Excerpt"
    :p (list stairway-rh stairway-lh)
    :tempo 98
    :time-signature '(4 4))



;; Option 2 -------------------
;
; This works a bit better
; Pros:
;      - rh and lh are less separated and therefore easier to reason about and compose
; Cons: 
;      - slash chords are not shown at all as symbols in score
;      - too much biolerplate in code

(setf stairway
      (append
       (chordize (append '(a2) (expand-chord '((a4 m)))))
       (chordize (append '(gs2) (expand-chord '((e4 maj)))))
       (chordize (append '(g2) (expand-chord '((c4 maj)))))
       (chordize (append '(fs2) (expand-chord '((d4 maj)))))
       (chordize (append '(f2) (expand-chord '((f4 maj7)))))
       ))
(ps 'gm
    :pg (list (make-omn :pitch stairway :length '(w) :span :pitch))
    :tempo 98
    :time-signature '(4 4))



;; Option 3 -------------------
;
; This IMHO would be ideal, but yet is not implemented in OM
; Just a proposal for notation, maybe there a better ways.
; Musicians please contribute!
; I believe the already supported symbols are great and
; I am not sure if my proposal is compatible.
;
; Pros:
;      - rh and lh are less separated and therefore easier to reason about and compose
;      - slash chords are shown as symbols in score
;      - less biolerplate in code

(setf stairway
      '(w (a4 m/a2)
          (e4 maj/gs2)
          (c4 maj/g2)
          (d4 maj/fs2)
          (f4 maj7/f2)))
=> (w (a4 m/a2) (e4 maj/gs2) (c4 maj/g2) (d4 maj/fs2) (f4 maj7/f2))

(ps 'gm
    :pg (list stairway)
    :tempo 98
    :time-signature '(4 4))
=> Error: OMN Parse Error: fail