What's in a powerset? A set's subsets!
An empty set is any set's subset,
so powerset of empty set's not empty.
Its (only) element it is an empty set:
(define
(powerset aL)
(cond
[(empty? aL) (list empty)]
[else
As for non-empty sets, there is a choice,
for each set's element, whether to be
or not to be included in subset
which is a member of a powerset.
We thus include both choices when combining
first element with smaller powerset,
that, which we get recursively applying
the same procedure to the rest of input:
(combine (first aL)
(powerset (rest aL)))]))
(define
(combine a r) ; `r` for Recursive Result
(cond
[(empty? r) empty] ; nothing to combine `a` with
[else
(cons (cons a (first r)) ; Both add `a` and
(cons (first r) ; don't add, to first subset in `r`
(combine ; and do the same
a ; with
(rest r))))])) ; the rest of `r`
"There are no answers, only choices". Rather,
the choices made, are what the answer's made of.
