an idea/extension could be to "filter-events" not only by singular pitches/lengths... but also by motifs, means sequences => "filter a PATTERN" 

function-name: filter-pattern

When you suggest something please in context of a function: input (var) and output (result).

This will enhance the communication. 

violà, take it as a sketch...

 

;;; like this, but the question is how to structure the input-values
;;; the kind of structure decides about output...

(defun filter-pattern (pattern omn-list)
  (progn
    (if (atom (car pattern))
      (setf pattern (mapcar #'list pattern)))
    (flatten 
     (loop 
       for i in (single-events omn-list)
       with cnt = 0
       
       when (pattern-matchp i (nth cnt pattern))
       collect i and do (incf cnt)
       else collect (neg! (omn :length i))
       
       when (= cnt (length pattern)) do (setf cnt 0)))))
  

(setf seq '(e c4 mp -e fermata e. d4 -h e. c4 e e4))


(filter-pattern (rnd-sample-seq 2 (single-events seq)) 
                '(e c4 mp -e fermata e. d4 -h e. c4 e e4))

(filter-pattern '((e. c4) (e4))
                '(e c4 mp -e fermata e. d4 -h e. c4 e e4))

(filter-pattern '((c4) (e4))
                '(e c4 mp -e fermata e. d4 -h e. c4 e e4))

(filter-pattern '(c4 e4)
                '(e c4 mp -e fermata e. d4 -h e. c4 e e4))