[racket] set! racket behaviour

Hi,

I’m using DrRacket. 
In an edit window I have written:

#lang racket
(define counter 0)

I’m pressing “Run” button and next, in a command line, after running a command:

(set! counter (add1 counter))

I receive a message “set!: cannot modify a constant: counter”

The problem can be resolved by changing the language (Language/Choose language/R5RS and unchecking “Disallow redefinition of initial binding”) but I suppose that in this way I can lose an access to the racket libraries.
Of course, I realize, that redefinition of the variables is not good (functional) programming style but sometimes it’s necessary. Is there a possibility of such modifications without changing language?

Jerzy

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