ed(1) is the standard editor.

ed(1)term is a Raspberry Pi running Plan 9 which (optionally)
boots directly to ed(1) for a distraction-free writing environment.
Under more conventional boot situations, it will also monitor a
gpio pin and open a new window running ed(1) when the pin is
grounded.

The boot behavior is driven by a change to /rc/bin/termrc.local
(from the comment "# ed(1)term" to EOF), while the monitoring
is provided by 'edwin', run by (n)rio's -i. Both rely on 'pibutton',
a trivial program which prints the state of each gpio pin.

Both scripts monitor pin 5 (logical, not physical, pins; that's pin
29 on all models after the first), and expect the switch to run to
ground. Both of these are trivial to change.