Support User-Defined Node Expression Functions

[HolgerKnublauch]

Not sure if this was captured by another ticket but we had discussed the ability for SHACL users to define new Node Expression functions that are calling other functions under the hood. This would basically be parameterized functions that use sh:var to access input values in certain places.

[afs]

Example?

[afs]

Is this related to currying functions?
Or function definition?

# [ :myInc (3) ] ==> 4
:myInc sh:defun [
    sh:args ( [ sh:var "x" ] ) ;
    sh;functionBody [ sparql:plus ( [ sh:var "x" ] 1 ) ] ]
  ] .

taking inspiration from other places.

[afs]

Collecting related issues and comments:

• Support User-Defined Node Expression Functions #498
• Sequence processing #484
• NodeExp composability / partial evaluation / parametrization #502
• Initial content for 'Inference Rules' #452 (comment)

Feel free to edit and add bullet points.