environment model explanation

to make sense of down2, carefully evaluate its definition

• let is short for lambda
• so we evaluate in the top-level environment
  ((lambda (x) )) (lambda (i) (set! x (- x i)) x)) ) 100)
  
• create a fresh frame for the argument x
• the inner lambda evaluates to itself, since it’s not applied here
• resulting value is bound to down2 in top-level env
• now when down2 is applied, evaluation of body is in the environment EB

x : 100down1 :down2 :

param ibody (begin (set! x (- x 1)) x))

x : 100

param ibody (begin (set! x (- x 1)) x))

EB

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