Mark current point as the beginning of an abbrev.
Abbrev to be expanded starts here rather than at beginning of word.
This way, you can expand an abbrev with a prefix: insert the prefix,
use this command, then insert the abbrev. This command inserts a
temporary hyphen after the prefix (until the intended abbrev
expansion occurs).
If the prefix is itself an abbrev, this command expands it, unless
ARG is non-nil. Interactively, ARG is the prefix argument.
(fn &optional ARG)