In this case, the code is much much simpler than your patch:
(defun Markus-filter (pitch lower upper)
(remove nil
(loop for i in pitch
collect (if (and (<= i upper)
(>= i lower)) i))))
Here is the lisp code:
markus-filter.lisp (170 Bytes)
And you can even simplify your graphic code following this code:
here is the patch:
e.g.flûte_basse1 (15.1 KB)
Best
K