Great post, as always, Daniel. It's an odd corner case, but when you're talking about tie chains and manipulating their pitches as a group, will your program have consideration of "enharmonic ties" -- I've posted about this somewhere, but I first encountered them in a Clara Schumann Piano trio and have seen them a number of times since. Where, due to changing harmonies/keys/etc. (in other parts), a D# can be tied to an Eb. Musically, there's no contradiction to tying the two notes, but from an engraving perspective, it can be a nightmare unless the code can quickly switch to slur code in this case. Other more common issues are tied notes that cross a clef change or 8va mark change. (in these cases, I prefer two broken ties, one leading out of the first note and the second leading in, as if it's a tie over a system break, rather than using a slur mark). Thanks!