[syntax] set! variable expression

R5RS, R6RS: 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.

[syntax] set! (proc arg ...) expression

SRFI-17: A syntax sugar of the following expression:

 ((setter proc) arg ...) expression)

See also setter, getter-with-setter.