Return non-nil if the selected window is the only window.
Optional arg NOMINI non-nil means don't count the minibuffer
even if it is active. Otherwise, the minibuffer is counted
when it is active.
Optional argument ALL-FRAMES specifies the set of frames to
consider, see also `next-window'. ALL-FRAMES nil or omitted
means consider windows on the selected frame only, plus the
minibuffer window if specified by the NOMINI argument. If the
minibuffer counts, consider all windows on all frames that share
that minibuffer too. The remaining non-nil values of ALL-FRAMES
with a special meaning are:
- t means consider all windows on all existing frames.
- `visible' means consider all windows on all visible frames on
the current terminal.
- 0 (the number zero) means consider all windows on all visible
and iconified frames on the current terminal.
- A frame means consider all windows on that frame only.
Anything else means consider all windows on the selected frame
and no others.