Function: exchange-dot-and-mark

Put the mark where point is now, and point where the mark is now.
This command works even when the mark is not active,
and it reactivates the mark.

If Transient Mark mode is on, a prefix ARG deactivates the mark
if it is active, and otherwise avoids reactivating it. If
Transient Mark mode is off, a prefix ARG enables Transient Mark mode temporarily.