When tweaking Tie grobs, the music function \alterBroken has
incorrectly rejected them since 2.17.6 as not being spanners.
The method of spanner recognition is at fault: 'span-direction
is not used by TieEvent.
Also, use eqv? instead of eq? for comparison of numbers.
form of a spanner event, @var{property} may also have the form
@samp{Grob.property} for specifying a directed tweak.")
(if (ly:music? item)
- (if (eq? (ly:music-property item 'span-direction) START)
+ (if (or (eqv? (ly:music-property item 'span-direction) START)
+ (music-is-of-type? item 'tie-event))
(tweak property (value-for-spanner-piece arg) item)
(begin
(ly:music-warning item (_ "not a spanner"))