Lazy in racket

How2lazy in DrRacket?

There's the lazy library, but it's not functional, i.e. I define a to be 1, define b as a + a, force b, it gives 2, but then I force set a to be 5, and it gives b as 2 again.

Any way around this?

>>7
Ah, I guess there's a difference between ((lambda (b) b) (+ a a)) and (define (b) (+ a a))