— syntax: set! <variable> <expression>

<Expression> is evaluated, and the resulting value is stored in the location to which <variable> is bound. <Variable> must be bound either in some region enclosing the set! expression or at top level. The result of the set! expression is unspecified.

(define x 2)
     (+ x 1)                                ==>  3
     (set! x 4)                             ==>  unspecified
     (+ x 1)                                ==>  5