#lang plait
(define last-call
(let ([state (none)])
(lambda (n)
(let ([old state])
(begin
(set! state (some n))
old)))))
(test (last-call 1) (none))
(test (last-call 2) (some 1))
(test (last-call 3) (some 2))
(test (last-call 3) (some 3))