bug#33309: Add flatten-list?

[martin rudalics]

+(defun flatten-tree (tree)
+  "Take TREE and \"flatten\" it.
+This always returns a list containing all the elements of TREE.
+Dotted pairs are flattened as well, and nil elements are removed.

Could you please sketch here (1) the expected form of TREE, (2) in
which order (pre-order I presume) it is traversed and therefore the
resulting list constructed and, as Stephen already asked, (3) which
elements of TREE will be included in the resulting list.

If applicable, please also tell if there are elements that this
function cannot flatten and which they are.

Thanks for your work on this, martin