By god they are right, this might change the future of mathematics!

// 2024‑edition Rust
use std::rc::Rc;

/// Church numeral: given a successor `s: fn(u32) -> u32`,
/// returns a function that applies `s` n times.
type Church = Rc u32) -> Rc u32>>;

/// 0 ≡ λs.λx.x
fn zero() -> Church {
    println!("Define 0");
    Rc::new(|_s| Rc::new(|x| {
        println!("  0 applied to {}", x);
        x
    }))
}

/// succ ≡ λn.λs.λx. s (n s x)
fn succ(n: Church) -> Church {
    // `label` is printed *before* the closure is created, so the closure
    // does not capture any non‑'static reference.
    println!("Build successor");
    Rc::new(move |s| {
        // `inner` is the predecessor numeral applied to the same successor
        let inner = n(s);
        Rc::new(move |x| {
            // first run the predecessor
            let y = inner(x);
            println!("  predecessor applied to {} → {}", x, y);
            // then apply the extra successor step
            let z = s(y);
            println!("  +1 applied to {} → {}", y, z);
            z
        })
    })
}

/// Convert a Church numeral to a Rust integer, printing each step.
fn to_int(n: &Church) -> u32 {
    let inc: fn(u32) -> u32 = |k| {
        println!("    inc({})", k);
        k + 1
    };
    let f = n(inc);               // f: Rc u32>
    println!("  evaluate numeral starting at 0");
    f(0)
}

/// Even ⇔ divisible by 2
fn is_even(n: &Church) -> bool { to_int(n) % 2 == 0 }
fn is_odd(n: &Church) -> bool  { !is_even(n) }

fn main() {
    // ---- build the numerals step‑by‑step ----
    let zero = zero();                     // 0
    let one  = succ(zero.clone());         // 1 = succ 0
    let two  = succ(one.clone());          // 2 = succ 1

    // ---- show the numeric values (trace) ----
    println!("\n--- evaluating 0 ---");
    println!("0 as integer → {}", to_int(&zero));

    println!("\n--- evaluating 1 ---");
    println!("1 as integer → {}", to_int(&one));

    println!("\n--- evaluating 2 ---");
    println!("2 as integer → {}", to_int(&two));

    // ---- parity of 2 (the proof) ----
    println!("\n--- parity of 2 ---");
    println!("Is 2 even? {}", is_even(&two)); // true
    println!("Is 2 odd?  {}", is_odd(&two));  // false

    // Proof: “divisible by 2” ⇔ “even”.
    // Since `is_odd(&two)` is false, no odd number can satisfy the
    // divisibility‑by‑2 condition.
    assert!(!is_odd(&two));
    println!("\nTherefore, no odd number is divisible by 2.");
}