- prefixes numbers along the number line in the opposite direction. If we had a second branch starting after 3 we could do the same if they shared digits and bases with the standard number line. Maybe suffix - or prefix 3№ or something like that.
Clearly XKCD doesn't time travel because Mathmaticians who create yet another use for the Delta symbol would be crucified in the future Meritocratic Dystopia.
You can do something like this with ℝ[ω] where ω is a complex cube root of unity and the restriction that at least one of the real or ω-part of a+bω must be zero. Edit: Just realised that I didn't specify that a and b should be non-negative reals, otherwise we end up with six lines from 0 not three.
Standard addition breaks immediately, but an addition-like operator can be defined by imagining a train shunting along the restricted lines in the direction of the greater magnitude.