N is not always closed, and that, more generally, the union of a closed set and a IIn0-set is not always IIn+10. In the proof of this fact we make use of the intuitionistic Borel Hierarchy Theorem, established in (Veldman, 1981) and (Veldman, 2001a)." />
On the persistent difficulty of disjunction - Veldman Wim | sdvig press
