Toggle Iswitchb mode.
With a prefix argument ARG, enable Iswitchb mode if ARG is
positive, and disable it otherwise. If called from Lisp, enable
the mode if ARG is omitted or nil.
Iswitchb mode is a global minor mode that enables switching
between buffers using substrings. See `iswitchb' for details.
(fn &optional ARG)
Non-nil if Iswitchb mode is enabled.
See the command `iswitchb-mode' for a description of this minor mode.
Setting this variable directly does not take effect;
either customize it (see the info node `Easy Customization')
or call the function `iswitchb-mode'.