Adding some new operators - Mailing list pgsql-hackers

Hi,

I am trying to use postgresql in some research
project.  I need to add some new operators, some
will probably take more than 2 input tuple streams.
The new operator will be considered by optimizer
when estimating cost and choosing a plan.

Can anyone tell me how hard this will be?  And
where is a good place to start? 

Thanks.

Feng