A pro-form is a word, substituting for other words, phrases, clauses, or sentences, whose meaning is recoverable from the linguistic or extralinguistic context.
But how do you establish a word as a proform and are there rules that guide the classification of a Pro-Form?
Schachter, Paul. 1985. "Parts-of-speech systems." In Shopen 1985b 24–25
Crystal, David. 1985.A dictionary of linguistics and phonetics. 2nd edition. New York: Basil Blackwell. 247