Applications can add SPARQL functions to the query engine. This is done by
writing a class implementing the right interface, then either registering it or
using the fake java:
URI scheme to dynamically call the function.
A SPARQL value function is an extension point of the SPARQL query language that allows URI to name a function in the query processor.
In the ARQ engine, code to implement function must implement the interface
com.hp.hpl.jena.query.function.Function
although it is easier to
work with one the abstract classes for specific numbers of arguments like
com.hp.hpl.jena.query.function.FunctionBase1
for one argument functions.
Function do not have to have a fixed number of arguments.
Function registry : global and per-query execution.