(set! ⟨variable⟩ ⟨expression⟩) ¶Semantics: ⟨Expression⟩ is evaluated, and the resulting value
is stored in the location to which ⟨variable⟩ is bound. It is an
error if ⟨variable⟩ is not bound either in some region enclosing
the set! expression or else globally. The result of the
set! expression is unspecified.
(define x 2) (+ x 1) ⇒ 3 (set! x 4) ⇒ unspecified (+ x 1) ⇒ 5