(define (tree-min t)
  (cond [(leaf? t) 0]
        [(not (number? (node-val t))) false]
        [(let ((min (node-val t))
               (min-left (tree-min (node-left t)))
               (min-right (tree-min (node-right t))))
           (cond [(and (<= min min-left) (<= min min-right)) min]
                 [(and (<= min-left min) (<= min-left min-right)) min]
                 [else min-right]))]))