Toggle Abbrev mode in the current buffer.
With a prefix argument ARG, enable Abbrev mode if ARG is
positive, and disable it otherwise. If called from Lisp, enable
Abbrev mode if ARG is omitted or nil.
In Abbrev mode, inserting an abbreviation causes it to expand and
be replaced by its expansion. (fn &optional ARG)
Non-nil if Abbrev mode is enabled.
Use the command `abbrev-mode' to change this variable.