Looking for information about rtree index

I'm working on a project that deals with geometrical information.
The R-tree indexing in Postgresql looks as if it might be useful.
I'm trying to figure out how to get my own data types to use R-trees.
The Server Programming document chapter 17 contains a partial example
of how to do this for a B-tree index, but I feel as if I've hit a dead
end. It assumes some information, such as the number, used and names
of the required access method strategies and support routines. Is
there somewhere I can look to find the needed information for R-trees?

* Patrick L. Nolan *
* W. W. Hansen Experimental Physics Laboratory (HEPL) *
* Stanford University *