A Scheme interpreter that passes only relevant subsets of environments to eval
(let ([x 1]
      [y 2])
  (+ x 3))

(let ([x 1]
      [y 2])
  (lambda (z)  ;; free vars = {x}
    (+ x z))

(let ([x some-big-object])
  (f (g x) long-running-expression-not-involving-x))

(let ([x some-big-object])
  (let ([tmp1 (g x)])
    (set! x #f)
    (let ([tmp2 long-running-expression-not-involving-x])
      (f tmp1 tmp2))))