as a consequence, combinator k is not present in the λ calculus nor in the cl calculus. the constants of cl are: i, b, c and s, which form a basis from which all cl terms can be composed (modulo equality). every λ term can be converted into an equal cl combinator according to rules similar to those presented above for the conversion of λ terms into cl combinators. see chapter 9 in barendregt (1984).