public static ICanbe <I[]> FailOnEmpty <I>(this ICanbe <I[]> self, string message, int code = 0) => self.FailIf(xs => xs == null || (!xs.Any()), message, code);
public static ICanbe <I> FailOnNull <I>(this ICanbe <I> self, string message, int code = 0) => self.FailIf(xs => xs == null, message, code);