An omega-power of a context-free language which is Borel above Delta^0

Jacques Duparc Olivier Finkel An omega-power of a context-free language which is Borel above Delta^0_omega http://arxiv.org/abs/0801.1783 2008 CoRR abs/0801.1783 db/journals/corr/corr0801.html#abs-0801-1783 informal publication