The Solovay Theorem says:
Let $\kappa$ be a regular uncountable cardinal. Then every stationary subset is the disjoint union of $\kappa$-many stationary subsets. [Jech, Theorem 8.8, p. 95]
One step in Jech's proof of this theorem I do not understand (underlined in red in the picture below). Why is $D$ a closed unbounded set?
