Github user afs commented on the issue:
https://github.com/apache/jena/pull/273
This is just completing the coverage for F&O 3.1.
I don't think `fn:apply` replaces any custom function. There is a syntax
extension `CALL` (little know, undocumented; a toy from a long time ago) which
is very similar.
"FN_Collation" is related `fn:collation-key` has a specific, different
result type specified by the standard. (there isn't a `fn:collation` in
Functions and Operators 3.1 (F&O) so it is oddly named - it's wired to an ARQ
URI which is correct) Can I rename FN_Collation as "collation" to make the
naming systematic? (FN_* are F&O functions).
With `fn:apply` it would be nice is to be able to call `op:` things e.g.
`op:numeric-add` which aren't callable functions in F&O so by my reading,
aren't covered.
As all the operation code (`E_*`) are not inheriting from "Function", a bit
of adapter code is needed to register them. Some messing around with arities.
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