Run function FUN on the subtree of windows rooted at WINDOW.
WINDOW defaults to the selected window. FUN must be a function
with one argument - a window. By default, run FUN only on live
windows of the subtree. If the optional argument ANY is non-nil,
run FUN on all live and internal windows of the subtree. If
WINDOW is live, run FUN on WINDOW only.
This function performs a pre-order, depth-first traversal of the
subtree rooted at WINDOW. If FUN changes that tree, the result
is unpredictable.