R4RS 4.1.6  (set! <variable> <expression>)  ==>  unspecific

<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
