;recursive functional
(let ! (proc (n)
    (? (= n 0)
        1
        (* n (! (+ n (- 1))))
    )
))

;recursively-defined iterative functional
(let ! (proc (n)
    (let iter-fact (proc (a n acc)
        (? (= a n)
            (* acc n)
            (iter-fact (+ a 1) n (* a acc))
        )
    ))
    (iter-fact 1 n 1)
))

;iteratively-defined (procedural)
(let ! (proc (n)
    ;these are just INITIAL values, the loop cannot change them in this scope, it will bind to the same value in its own scope and change within that
    (let a 1)
    (let acc 1)
    (let loop (proc emp
        (? (= a n)
            (* acc n)
            (begin
                (let acc (* a acc))
                (let a (+ a 1))
                (loop)
            )
        )
    ))
    (loop)
))