#lang plait
(define-type BT
  [mt]
  [node (v : Number) (l : BT) (r : BT)])

(define (size-pm t)
  (type-case BT t
    [(mt) 0]
    [(node v l r) (+ 1 (+ (size-pm l) (size-pm r)))]))
(size-pm (mt))