I plugged a monitor into my laptop's VGA port.  I ran xrandr --auto
and got the mirrored screen.  I found it a little distracting, so I
turned off the LVDS: xrandr --output LVDS --off.  So far, so good.

Now I want to unplug the monitor.  How do I turn the laptop screen
back on?  I note that there is no --on option to match with --off.

atom:~> xrandr --output LVDS --auto
xrandr: Configure crtc 1 invalid time

Am I looking at a bug, user error, missing feature, what?