what is the purpose of table unit?

[Tim Harsch]

Hi,
I'm trying to write a transformer on a spatial property function.  For example:


The following algbra

(propfunc <http://jena.apache.org/spatial#withinBox>
        ?s (37.6315270872214 -122.52347356152336 37.81401629628255 
-122.35095387768547)
        (table unit)
      )

Comes in to:
public Op transform(OpPropFunc opPropFunc, Op subOp) 


as
(propfunc <http://jena.apache.org/spatial#withinBox>
        ?s (37.6315270872214 -122.52347356152336 37.81401629628255 
-122.35095387768547)
        (table unit)
      )
for opPropFunc

and
(table unit)
for subOp

I don't really understand why some transforms have a subOp and some don't