public static bool exists <L, R>(Either <L, R> either, Func <R, bool> Right, Func <L, bool> Left) => either.Exists(Right, Left);
public static bool exists <R, L>(Either <R, L> either, Func <R, bool> pred) => either.Exists(pred);
public static bool exists <L, R>(Either <L, R> either, Func <L, bool> pred) => either.Exists(pred);