The issue with tied notes it that they are ... tied. Let's look at the following example to understand what the issue actually is:

This music sheet means that the la key must be pressed only once, and for a total of 1.2s (two quarter time) However, when simply using the default event listener, the output we get is as follow:

0.00000000	tempo	400.00000000
0.00000000	note	69	4	0.25000000	point-and-click 4 36
0.00000000	tie
0.25000000	note	69	4	0.25000000	point-and-click 11 36

The event listener generates lines per ... event, and on the music sheet there are two notes, hence two lines. Instead of saying there should only be one key pressed, it says the key should be pressed twice, which is wrong.

My first approach to tackle this issue was to use the midi file. Lilypond can generate a midi file alongside the music sheet. As a rough estimate approximation, a midi file is akin to an mp3 file but for instrument music only. Since lilypond generates midi file that correctly describes how the music is played, it would be possible to know if a key is played longer than what is described in the events file, and thus if it is tied to the next key with the same pitch. This lead me to develop pianoterm. However some issues with this approach quickly appeared. When playing triolet for example, there is a slight difference in the timings outputed in the events file and the one in the midi files due to some rounding in the midi file. This could be easily fixed by correlating the timings, however this approach looked a bit too britle hence I looked for something else.

On the generated events above, we can see there is a tie event. We could use this as it means the previous note has a tie attached, but sadly this is not precise enough. For example, the following two equivalents music sheets provides the following two different events:

music sheet	lilypond source	generated events
	<la' sol'>4~ <la' sol'>
	<sol' la'>4~ <sol' la'>

On the first first music sheet, looking at the generated events only, it looks like the tie applies to the sol key, whereas on the second one it looks like it applies to the la key. Truth is, both the sol and la key have a tie attached.

For an even more contrived example, the two following distinct music sheets give the same tie events (as seen per the event-listener)

music sheet	Source	generated events
	<sol' la'~ mi''>4~ <sol' la' mi''>4
	<sol' la' mi''>4~ la'

One solution to overcome this issue would be to get on each line corresponding to a note, something saying if yes or no there is a tie attached to that note.

Since Lilypond provides the option to let the user include file for global settings, I used it to extract more data from the event-listener than the default one.

That is, I created a file named event-listener.scm with this content and then called lilypond using lilypond -dinclude-settings=event-listener.scm <music-sheet.ly>.

The following table shows the new event-listener's output:

music sheet	source	generated events
	<la' sol'>4~ <la' sol'>
	<sol' la'>4~ <sol' la'>

Sadly, on these generated events, it says no notes have a tie attached. On a positive side we can say that at least we got some consistency here. Two different inputs which are logically equivalent now generates the same output,

One way to workaround the issue, is to rewrite the music sheet to explicitly say that each note on the chord has a tie attached instead of saying the whole chord has a tie attached. Below show the difference in writing the input, and the generated output.

music sheet	source	generated events
	<la'~ sol'~>4 <la' sol'>
	<sol' la'~ mi''>4 la'
	<sol'~ la'~ mi''~>4 <sol' la' mi''>4

Now we properly get for each note if there is a tie attached to it. This solution is not satisfactory as it requires modifying by hand the input music sheet, but sadly I could not find anything better yet.