\version "2.19.46"

% angle brackets cause SVG error so reformat:
% #<Mom 1> becomes [Mom 1]
#(define (reformat-moment mom)
   (let* ((mom (format #f "~a" mom))
          (mom (string-drop mom 2))
          (mom (string-drop-right mom 1)))
     (string-concatenate (list "[" mom "]"))))

% grob name + absolute moment + (system) coordinates should be enough for a unique id...
#(define (get-unique-id grob)
   (let ((sys (ly:grob-system grob)))
     (format #f "~a/~a,(~a,~a)"
       (grob::name grob)
       (reformat-moment (grob::when grob))
       (ly:grob-relative-coordinate grob sys X)
       (ly:grob-relative-coordinate grob sys Y))))

assignIDs =
#(let ((grob-names (map car all-grob-descriptions)))
   #{
     #@(map (lambda (x)#{ \override #(list 'Score x 'id) = #get-unique-id #})
     grob-names)
   #})


{
  \assignIDs
  a1
  \break
  a
  \break
  a
}