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(define (func-for-nelon a-nelon)
    (cond
       \[(empty? (rest a-nelon))
           … (first a-nelon) …\ ]
       \[(cons? (rest a-nelon))
           … (first a-nelon) … (func-for-nelon (rest a-nelon)) …\ ]))

Solution 2:

We could use the definition of (perhaps-empty) list-of-numbers, that we've already seen.

; A non-empty-list-of-numbers is

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; a cons, which has a first, a number, and rest, a list-of-numbers

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; (cons num lon)

Here, the template is different, since it must refer to the template for a regular (non-empty) list-of-numbers. In particular, you don't make a recursive call; you think to yourself "Ah, a-nelon contains a (perhaps-empty) list; I'd better call a function for such regular ol' lists":

(define (f a-nelon)

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    … (first a-nelon) … (func-for-lon (rest a-nelon)) … )

Here, to compute the average, what function(s) of general (perhaps-empty) lists do you want to call, as your helper?

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