Function S 

pub fn S() -> Term

S - the substitution combinator.

S ≡ λxyz.x z (y z) ≡ λ λ λ 3 1 (2 1)

Example

use lambda_calculus::combinators::S;
use lambda_calculus::*;

assert_eq!(
    beta(app!(S(), Var(1), Var(2), Var(3)), NOR, 0),
    app!(Var(1), Var(3), app(Var(2), Var(3)))
);