The Little Schemer evens-only*&co

Question

I\'m having difficulty understanding what\'s going on with The Little Schemer\'s evens-only*&co example on page 145.

Here\'s the code:

Answer 1

The answer by Jon O. is a really great in-depth explanation of underlying concepts. Though for me (and hopefully, for some other people too), understanding of concepts like this is a lot more easier when they have a visual representation.

So, I have prepared two flow-charts (similar to ones I did for multirember&co, untangling what is happening during the call of evens-only*&co

given l is:

'((9 1 2 8) 3 10 ((9 9) 7 6) 2) 

and col is:

(define the-last-friend
    (lambda (newl product sum)
        (cons sum (cons product newl))
    )
)

One flow-chart, reflecting how variables relate in different steps of recursion: Second flow-chart, showing the actual values, being passed:

My hope is, that this answer will be a decent addition to the Jon's explanation above.