Quoting Steve Litt (slitt@???):

> Does Post Office work properly with IMAP?
[...]
> What is toggletree?

Found it. Fl_ToggleTree is described as 'A Tree Browser Widget for FLTK'.
http://www.fltk.org/wiki.php?V79+TC+Q

Note that according to poster 'lm' on that page, source code for
Fl_ToggleTree is included in the tarball for Post Office.

On http://www.fltk.org/wiki.php?V43+TC+Q, Post Office is reported to be
buildable with FLTK 1.3.2. (Perhaps that is where you saw the reference
to 'toggletree'.)