1986, Raymond Hickey, “Possible phonological parallels between Irish and Irish English†in English World-Wide VII, № 1, page 9, § 1.6.2:While and lost their distinctiveness in pre-rhotic, tautosyllabic positions, the basic distinction back ~ front was maintained in forms such as the following: fern , General IrE ; turn , General IrE .
2001, James Harris, “Spanish Negative in-: Morphology, Phonology, Semanticsâ€, chapter 5 in Kenneth L. Hale: A Life in Language, ed. , MIT Press, ISBN 0262112574 (hbk), ISBN 0262611600 (pbk), page 179:We turn now to the prerhotic context in (15)–(17).