Function head 

pub fn head() -> Term

Applied to a pair-encoded list it returns its first element; equivalent to pair::fst.

HEAD ≡ λp.p TRUE ≡ λ 1 TRUE ≡ FST

Example

use lambda_calculus::data::list::pair::head;
use lambda_calculus::*;

let list = vec![1.into_church(), 2.into_church(), 3.into_church()].into_pair_list();

assert_eq!(beta(app(head(), list), NOR, 0), 1.into_church());