Method: (P-DELETE-IF T PERSISTENT-CONS)

Source

(defmethod p-delete-if (test (list persistent-cons)
                        &key (from-end nil) (start 0) end count key)
  ;; DO: Implement FROM-END.
  ;; DO: Write tests.
  (declare (ignore from-end))
  (unless key
    (setq key #'identity))
  ;; Move list to start position.
  (let ((tail list)
        (prev nil))
    (loop repeat start
          do (setq prev tail
                   tail (p-cdr tail)))
    ;; The real work.
    (let ((nr-deleted 0))
      (loop for i from start do
            (if (or (p-endp tail)
                    (and end (= i end))
                    (and count (>= nr-deleted count)))
                (return-from p-delete-if list)
              (if (funcall test (funcall key (p-car tail)))
                  ;; Delete the element.
                  (progn
                    (if prev
                        (setf (p-cdr prev) (p-cdr tail))
                      (setq list (p-cdr tail)))
                    ;; Keep count.
                    (incf nr-deleted))
                ;; Don't delete anything.
                (setq prev tail)))
            ;; Keep moving.
            (setq tail (p-cdr tail)))))
  ;; Return the (possibly modified) list.
  list)