"BGP" vs. "Reorder" in INFO log messages

[Bob DuCharme]

When I execute the following query with ARQ and the -v parameter,
   PREFIX : <http://some/uri/>

   SELECT * WHERE {
     OPTIONAL { ?s :comment ?comment . }
     ?s :firstName ?firstName ; :lastName ?lastName .
   }

I see the following in the output:

  11:53:02 INFO  exec     :: BGP :: (?s <http://some/uri/firstName> 
?firstName) (?s <http://some/uri/lastName> ?lastName)
  11:53:03 INFO  exec     :: Reorder :: (?s <http://some/uri/firstName> 
?firstName) (?s <http://some/uri/lastName> ?lastName)
  11:53:03 INFO  exec     :: BGP :: (?s <http://some/uri/comment> ?comment)
  11:53:03 INFO  exec     :: Reorder :: (?s <http://some/uri/comment> 
?comment)

I can see the reordering that was done (especially if I rerun the query 
with optimize=off) but can someone tell me or point me to documentation 
of what "BGP" vs. "Reorder" mean in these INFO messages?

Thanks,

Bob