+ (if (equal? (object-property sym 'backend-type?) #f)
+ (begin
+ (ly:warning (_ "cannot find property type-check for ~a") sym)
+ (ly:warning (_ "doing assignment anyway"))))
+ (set!
+ (ly:music-property arg 'tweaks)
+ (acons sym val
+ (ly:music-property arg 'tweaks)))
+ arg)