SET!

SET! is a special form that evaluates its second argument and assigns that value to the defined variable which must be its first argument.

> (define x 0)

> (set! x (+ 1 2 3))

> x

6

> (define y 0)

> (set! y ’(+ 1 2 3))

> y

(+ 1 2 3)

> (set! y (rest y))

> (set! y (first y))

> y

1

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