let nil = Node.make Nil
let cons a b = Node.make (Cons(a,b))
let hd = function { Node.node = Cons(a,_) } -> a | _ -> failwith "hd"
let nil = Node.make Nil
let cons a b = Node.make (Cons(a,b))
let hd = function { Node.node = Cons(a,_) } -> a | _ -> failwith "hd"