i wanted to include your engraver in Fried Songs project, but there's an error. Unfortunately i don't have time to debug this problem at the moment, but maybe the error message will give you an idea what is happening?
In procedure string= in expression (string= (ly:prob-property # #) "staccato"): Wrong type argument in position 1 (expecting string): ()
I've just pasted the definition and context modification to main .ly file... When i used it as a separate snippet everything was fine.