Maaan, I just sat down, downloaded your system and rewrote all the function names and fixed Feather's errors. Started saving the file, wanted to look up your nickname to put in the title and found that you had just updated the asset...
Viewing post in Verlet integration for GMS2.3+ comments
I added one more function for VIObject to be able to delete self when object is destroyed. Since I didn't find a corresponding function for this. The function I added does the following:
var _ind = ds_list_find_index(systemReference.objectList, self);
ds_list_delete(systemReference.objectList, _ind);
Did I do it right? Everything works fine for me!
If it works, it works :D There is still some functionality one could add like the delete function you've added, or interaction between different ropes/softbodies. However, I won't be able to work on this project for a longer time again now so feel free to add what you need :)
That being said, I plan to upload the project to github the next couple of days, so if you want you could also contribute there.