bug#33309: Add flatten-list?

[Stefan Monnier]

> For the records, in Tramp I need also this function. I'm just going to
> define it myself (derived from eshell-flatten-list), but I'll happily
> switch to a common flatten-list.

If you do, please use an appropriate name: if it recurses arbitrarily, it
means it's flattening a *tree* not a *list*.  IOW if we want to provide
`list-flatten` (or `seq-flatten`) it would have to look a bit like

    (apply #'append list)

without the recursion.

> Unless nobody else objects, I'd like to encourage you to prepare a patch
> to be added to the master branch. This could include the replacement of
> the existing implementations.

Agreed.  Just make sure the name clarifies it's working on a tree
(e.g. depending on how you define the function, it can be considered as
a function that returns all the leaves of a given tree).


        Stefan