marking tuples - Mailing list pgsql-sql

Hi,
I am implementing a new algorithm inside postgresql for a side research project. I am just wondering if anyone may give me few hints about the following problem:

Consider the following plan:

NewAggregate
    ->NewJoin
          ->NewJoin
                ->NewJoin
                       ->SeqScan
                       ->SeqScan
                ->SeqScan
          ->SeqScan


In my algorithm,
each NewJoin node may "mark" its ps_ResultTupleSlot with either red/green. I need that mark to flow in the pipeline so that i can collect them at the top aggregate.

Any hints about how to keep such mark is greatly appreciated.

Thanks a lot,
Ehab