let t12 = (make\_list t1 (make\_list t2 empty)) in
let t23 = (make\_list t2 (make\_list t3 empty)) in
let ta = (make\_list t1 t23) in
let t12 = (make\_list t1 (make\_list t2 empty)) in
let t23 = (make\_list t2 (make\_list t3 empty)) in
let ta = (make\_list t1 t23) in