An omega-power of a context-free language which is Borel above   Delta^0_omega
  We 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^\omega where 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. 
Jacques Duparc (UNIL), Olivier Finkel (LIP)