Revised(5) Report on the Algorithmic Language Scheme - Assignments

### 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