/// <summary>
/// Folds the either into an S
/// https://en.wikipedia.org/wiki/Fold_(higher-order_function)
/// </summary>
/// <typeparam name="S">State</typeparam>
/// <typeparam name="L">Left</typeparam>
/// <typeparam name="R">Right</typeparam>
/// <param name="self">Either to fold</param>
/// <param name="state">Initial state</param>
/// <param name="Right">Right fold function</param>
/// <param name="Left">Left fold function</param>
/// <returns>Folded state</returns>
public static S Fold <L, R, S>(this EitherUnsafe <L, R> self, S state, Func <S, R, S> Right, Func <S, L, S> Left) =>
self.IsBottom
? state
: self.IsRight
? self.Fold(state, Right)
: self.Fold(state, Left);