 Gurevich (2011) maintains, on the one hand, that it is not possible to provide formal definitions of algorithms, as the notion continues to evolve over time: consider how sequential algorithms, used in ancient mathematics, are flanked by parallel, analog, or quantum algorithms in current computer science practice, and how new kinds of algorithms are likely to be envisioned in the near future. On the other hand, a formal analysis can be advanced if concerned only with classical sequential algorithms. In particular, Gurevich (2000) provides an axiomatic definition for this class of algorithms.