module Rev;

def List<A> append<A>(List<A> xs, List<A> ys) =
    case xs {
    Nil => ys;
    Cons(x,xsP) => Cons(x, append(xsP, ys));
};

def List<A> rev<A>(List<A> xs) =
    case xs {
    Nil => Nil;
    Cons(x,xsP) => append(rev(xsP), Cons(x,Nil));
};

def List<A> start<A>(List<A> xs) = rev(xs);

{

}