#lang scheme
;; data definitions 

(define id? symbol?)
(define id-equal? eq?)
(define-struct basic-customer (id name address) #:mutable)

;; interface 
(provide/contract 
 [id?                   (-> any/c boolean?)]
 [id-equal?             (-> id? id? boolean?)]
 [struct basic-customer ((id id?)
                         (name string?)
                         (address string?))])
;; end of interface