I ran the example from the documentation
for graphics:
g = new graphics.

However in the units.txt datafile,
g is used for the gram units.

Is there some way to revert / reset
a variable (in this case, g)
to the default unit setting,
without closing and reopening the program?

I see that I can do this manually:
g = gram
But if the default isn't known
or is long (like R =
8.314472 m^2 s^-2 kg K^-1 mol^-1 (molar_heat_capacity)
), that's not so easy.

Is there some way to warn that a variable
is already assigned, so it isn't accidentally
changed (like g from grams to a graphics object)?