Syntax: Mark [0|1]
Abbreviation: M
sets the mark at the current position or cancels the previous mark.
The mark and cursor together define the range of text over which clips
(Cut, Copy, Erase) and left and right shifts operate.
If you invoke Mark with no arguments, it will set the mark. If you
specify 0 or 1, the mark will be cancelled or set to the current position,
respectively. A capital ‘M’ appears on the status bar, if the mark is
active.