module Flip;

data XX<A> = E | Z(XX<A>) | O(XX<A>);

def XX<A> flip<A>(XX<A>  x) =
    case x {
    E => E;
    Z(y) => O(flip(y));
    O(y) => Z(flip(y));
};

def XX<A> start<A>(XX<A> w) = flip(w);

{

}