Branislav Rovan L'ubos Steskal Infinite Computations and a Hierarchy in <i>Delta</i> <sub>3</sub>. 660-669 2007 conf/cie/2007 CiE http://dx.doi.org/10.1007/978-3-540-73001-9_68 db/conf/cie/cie2007.html#RovanS07