 One obstacle faced by the mathematician attempting to come to grips with RUSS is that, being expressed in the language of recursion theory, it is not easily readable; indeed, on opening a page of Kushner’s excellent lectures [1985], one might be forgiven for wondering whether this is analysis or logic. (This remark should be tempered with reference to the two relatively readable books on classical recursive analysis by Aberth [1980, 2001].) Fortunately, one can get to the heart of RUSS by an axiomatic approach due to Richman [1983] (see also Chapter 3 of Bridges &amp; Richman [1987]).