How to get bounding NoteColumns of the first of consecutive TextSpanners

[Thomas Morley]

Hi,

please have a look at the following minimal, producing three staves,
the middle one with consecutive TextSpanners:

mus = { bes \startTextSpan b\stopTextSpan \startTextSpan bis\stopTextSpan }
<< { a4 a a } \mus { c' c' c' } >>

Now I want to know the starting and ending NoteColumns of the _first_
TextSpanner.
Though, the first TextSpanner is left-bounded by NoteColumn and
right-bounded by PaperColumn.

How to get the NoteColumn at first TextSpanner's end?

Below some test-code.
Obviously I can filter PaperColumn's elements for NoteColumns, but
PaperColumn is a Score-grob, thus I get all three.
How to select?
(I may not know the position of the Staff with the TextSpanners)

#(define tst
  (lambda (grob)
    (let* ((left-bound (ly:spanner-bound grob LEFT))
           (right-bound (ly:spanner-bound grob RIGHT))
           (ncs-from-grob
             (ly:grob-array->list (ly:grob-object grob 'note-columns)))
           (right-bound-elts
             (ly:grob-array->list
               (ly:grob-object right-bound 'elements)))
           (ncs-from-right-bound
             (filter
               (lambda (elt) (grob::has-interface elt 'note-column-interface))
               right-bound-elts))
               )
    (pretty-print
      (list
        left-bound
        right-bound
        ncs-from-grob
        (equal? left-bound (car ncs-from-grob))
        ncs-from-right-bound
        )))))

mus = {
  bes4-\tweak #'after-line-breaking #tst
   \startTextSpan
  b\stopTextSpan
   \startTextSpan
  bis\stopTextSpan
}

<<
  { a4 a a }
  \mus
  { c' c' c' }
>>

Thanks,
  Harm