I've been thinking about the semantics of maptable. Right now it seems misnamed; it iterates over the table before returning it unmodified. But what should the right semantics be? Should it return a table with modified values for the same set of keys? Or should it return a list? Or should each iteration return a (k v) pair so you can get a new table with entirely new keys (I think map-as does this)? All of these could be useful; I think we need a more elaborate language than just map/fold to describe them.