Prolog-like append/3 for Hyperon in MeTTa

gistfile1.txt

use hyperon::metta::*;
use hyperon::metta::interpreter::*;

fn main() {
    let space = metta_space("
            (= (append () $L $L)  True)
            (= (append (Cons $H $T) $L (Cons $H $R)) (append $T $L $R))


    ");
    println!("{:?}", interpret(&space, &metta_atom("(append (Cons $A (Cons 2 $L1)) $L2 (Cons 1 (Cons $B (Cons 2 (Cons 3 (Cons 4 ()))))))")));
}

!(if (append (Cons $A (Cons 2 $L1)) $L2 (Cons 1 (Cons $B (Cons 3 (Cons 4 (Cons 5 ())))))) (print $A $B $L1 $L2) nop)

[(print 1 2 (Cons 3 (Cons 4 (Cons 5 ()))) ()),
(print 1 2 (Cons 3 (Cons 4 ())) (Cons 5 ())),
(print 1 2 (Cons 3 ()) (Cons 4 (Cons 5 ()))),
(print 1 2 () (Cons 3 (Cons 4 (Cons 5 ()))))]

?- append(
     [A,2|L1], L2,
      [1,B,3,4,5]).

A = 1, B = 2, L1 = [], L2 = [3,4,5] ; 
A = 1, B = 2, L1 = [3], L2 = [4,5];
A = 1, B = 2, L1 = [3,4], L2 = [5];
A = 1, B = 2, L1 = [3,4,5], L2 = [];