define the combinator S = lambda x.lambda y.lambda z. x z (y z). Note that the polymorphic types inferred for these terms correspond to propositional tautologies (read 'a -> 'a as "a implies a").

define the combinator    S = lambda x.lambda y.lambda z. x z (y z).   Note that the polymorphic types inferred for these terms correspond to propositional tautologies (read 'a -> 'a as "a implies a").