In "A Basis for a Mathematical Theory of Computation"* by McCarthy, which dates from the 1961 Western Joint Computer Conference, McCarthy mentions that Y can be used to eliminate LABEL but at the expense of giant resulting expressions. Who did invent Y, anyways? ------------ * Reprinted in "Computer Programming and Formal Systems", P. Braffort & D. Hirschberg eds, 1967. See pp 46-7.