I believe you mean "invariant with regard to future data in the time domain", i.e. f(x_1,..,x_t) == f(x_1,..,x_t,x_{t+1}) "Idempotent" means that f(f(x)) == f(

I believe you mean "invariant with regard to future data in the time domain", i.e. f(x_1,..,x_t) == f(x_1,..,x_t,x_{t+1}) "Idempotent" means that f(f(x)) == f(x), which wouldn't apply unless the output of a given analysis had to be fed back into it. Most outputs are going to be tables of counts with the input being raw text data, so that wouldn't apply in this case.