Re: Grace notes shouldn't shorten the previous note unless they overlap with it. (issue 29550043)

[dak]

https://codereview.appspot.com/29550043/diff/1/lily/note-performer.cc
File lily/note-performer.cc (right):

https://codereview.appspot.com/29550043/diff/1/lily/note-performer.cc#newcode100
lily/note-performer.cc:100: tie_head->length_mom_ = min
(tie_head->length_mom_,
Ok, I've brooded a lot over this line, and I'm still not sure what it
does in all cases.  Can you put a comment in here breaking down the
possible scenarios that this code can encounter?

The precondition apparently is now_mom ().grace_part < 0, and it's not
clear to me what grace parts tie_head->audio_column_->when () may have
for what reason.

now_mom () - start only makes sense when now_mom and start only differ
in _either_ grace_part _or_ main_part, and it's not clear to me whether
tie_head->length_mom_ can be a grace duration.

So please break this down somewhat more.  It's ok if this significantly
decreases the original code/comment ratio of this file: in all deference
to the original LilyPond authors, that's one aspect of the original code
style we are not trying to copy slavishly.

It probably took you quite a while to figure this out, so it's just
efficient to save the next person running across this code the time.

Thanks

https://codereview.appspot.com/29550043/