add tk when you hit a wall (abbreviated from 'to come', yet spelled with k as tc appears in many words)

But I can be wrong (maybe it's not from Doctorow, maybe the article did not even claim the paternity of coming up with TK but it was me badly understanding it, ...)

This assumes you're writing according to guidelines that insist you spell out all numbers. i.e. 58 is always intentionally "fifty-eight", so "58" must be your own meta text.