Various folds in the containers package

[Jon Fairbairn]

Duncan Coutts <duncan.coutts at googlemail.com> writes:

> So I suggest all four folds. They are all useful and can all be
> implemented efficiently.

For certain classes of operation ⓧ, a tree-fold

(( _ ⓧ _) ⓧ (_ ⓧ _))

gives better complexity. Is there room for that, or is it too
difficult to decide what to do about the unbalanced parts?

-- 
Jón Fairbairn                                 Jon.Fairbairn at cl.cam.ac.uk