Richard Chenevix Trench (1867). “On the Study of Words Lectures Addressed (originally) to the Pupils at the Diocesan Training-school, Winchester by Richard Chenevix Trench”, p.24
Andrea Moro, Noam Chomsky, Ivano Caponigro, Daniel B. Kane (2015). “The Boundaries of Babel: The Brain and the Enigma of Impossible Languages”, p.37, MIT Press