4.1.6 Assignments

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