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meteorgan

;; unbound variable in current frame (define (unbound? expr) (tagged-list? expr 'unbound)) (define (unbind-variable expr env) (make-unbound (cadr expr) env)) (define (make-unbound variable env) (let ((vars (frame-variables (first-frame env))) (vals (frame-values (first-frame env)))) (define (unbound vars vals new-vars new-vals) (cond ((null? vars) (error "variable is not in the environment -- MAKE-UNBOUND" variable)) ((eq? (car vars) variable) (set-car! env (cons (append new-vars (cdr vars)) (append new-vals (cdr vals))))) (else (unbound (cdr vars) (cdr vals) (cons (car vars) new-vars) (cons (car vals) new-vals))))) (unbound vars vals '() '()))) ;; add this in eval ((unbound? expr) (unbind-variable expr env))

meteorgan