The Leviathan Function library is a set of extension functions present in dotNetRDF's SPARQL engine. It contains a useful set of numeric functions which would be useful to have available in core ARQ and would boost portability of queries between ARQ and dotNetRDF (note that dotNetRDF already supports ARQs extension functions).
See https://bitbucket.org/dotnetrdf/dotnetrdf/wiki/DeveloperGuide/SPARQL/Leviathan%20Functions for definition of functions
Note that some of these overlap with new XPath 3 functions but for portability purposes there is no reason not to support these as well since most are relatively trivial to implement. I will file a separate issue for supporting XPath 3 functions.