;; Append th element of lists that match predicate to the end of the collection list (mutated);
;; Return the appended collection

(defun collect-if (predicate list &optional collection)
  (loop for x in list
        when (funcall predicate x)
          do (if (null collection)
                 (setf collection (list x))
                 (nconc collection (list x)))
        finally (return collection)))

(defun collect-if (predicate list &optional collection)
  (loop with coll = (cons nil collection)
        for x in list
        when (funcall predicate x)
          do (nconc collection (list x))
        finally (return (cdr collection))))

(defun collect-if (predicate list &optional collection)
  (loop for x in list
        when (funcall predicate x)
          do (push x collection)
        finally (return collection)))