Expand previous word "dynamically".
Expands to the most recent, preceding word for which this is a prefix.
If no suitable preceding word is found, words following point are
considered. If still no suitable word is found, then look in the
buffers accepted by the function pointed out by variable
A positive prefix argument, N, says to take the Nth backward *distinct*
possibility. A negative argument says search forward.
If the cursor has not moved from the end of the previous expansion and
no argument is given, replace the previously-made expansion
with the next possible expansion not yet tried.
The variable `dabbrev-backward-only' may be used to limit the
direction of search to backward if set non-nil. See also `dabbrev-abbrev-char-regexp' and C-M-/. (fn ARG)