Example of a Specific Set

Question

Give an example of a set $X_o$ such that, for each N $\in$ $\mathbb{Z}$$^+$, there is a sequence {X$_n$}from n=0 to N with each X$_{n+1}$ $\in$ X$_n$.

I just can't think an an example for this scenario. I do know that there shouldn't be an upper bound on the length of this sequence though.

Answer 1

HINT: Consider any infinite ordinal.