I don't lisp, but `in-iterators` is badly named unless there's precedent elsewhere in the lisp community. I initially expected it to implement (Cartesian) `prod

I don't lisp, but `in-iterators` is badly named unless there's precedent elsewhere in the lisp community. I initially expected it to implement (Cartesian) `product` rather than what's usually called `chain` (though admittedly the former is less useful in a language where `for*` is directly possible). Note that `product` really needs optimizations depending on whether its inner iterators can be trusted to be repeatable or not. I also find it a bit fragile for iterator implementations to use 2 functions, rather than a single function returning `value | unique-end-of-iterator-sentinel` (there are at least 3 obvious ways to lay this out such that there are no values an iterator cannot produce).