locate.lisp

(defun locate (x xs)
  (let* ((r0 nil)
         (r1 nil)
         (r2 (substitute-if x
                            (lambda (a)
                              (when (< x a)
                                (setf r0 a r1 t)
                                t))
                            xs
                            :count 1)))
    (values r0 r1 r2)))

; CL-USER 32 > (locate 3 '(0 1 2 4 5))
; 4
; T
; (0 1 2 3 5)