public static Cond <A, B> Then <A, B>(this Cond <A, A> self, B value) => self.Select(_ => value);
public static Cond <A, B> Then <A, B>(this Cond <A, A> self, Func <B> f) => self.Select(_ => f());
public static CondAsync <A, B> Then <A, B>(this Cond <A, A> self, Task <B> value) => self.Select(_ => value);