1. Barendregt, H.P.: The Lambda Calculus: Its Syntax and Semantics. Studies in Logic and the Foundations of Mathematics, vol. 103. North-Holland (1985)
2. Böhm, C.: Personal Communication
3. Broda, S., Damas, L.: On combinatory complete sets of proper combinators. J. Funct. Program. 7(6), 593–612 (1997)
4. Church, A.: Combinatory logic as a semigroup. Bull. Am. Math. Soc. 43, 333 (1937)
5. Curry, H.B., Feys, R.: Combinatory Logic, vol. 1. North Holland (1958)