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(begin <expression[1]> <expression[2]> ...)syntax

The <expression>s are evaluated sequentially from left to right, and the value(s) of the last <expression> is(are) returned. This expression type is used to sequence side effects such as input and output.

(define x 0)

(begin (set! x 5)
       (+ x 1))                          ===>  6

(begin (display "4 plus 1 equals ")
       (display (+ 4 1)))                ===>  unspecified
  and prints  4 plus 1 equals 5

As an extension to R5RS, CHICKEN also allows (begin) without body expressions in any context, not just at toplevel. This simply evaluates to the unspecified value.