2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/168661We use erasers-like basic operations on words to construct a set that is both Borel and above Delta^0_omega, built as a set V^ωwhere V is a language of finite words accepted by a pushdown automaton. In particular, this gives a first example of an omega-power of a context free language which is a Borel set of infinite rank.To appear in the Proceedings of the International Conference Foundations of the Formal Sciences V : Infinite Games, November 26th to 29th, 2004, Bonn, Germany, Stefan Bold, Benedikt Löwe, Thoralf Räsch, Johan van Benthem (eds.), College Publications at King's College (Studies in Logic), 2007Computational ComplexityComputer Science and Game TheoryLogic in Computer ScienceLogictext