custom index

Hi ,

i want to develop an index for a special problem.
The postgres docs shows the necessary bits in
http://www.postgresql.org/docs/8.4/interactive/indexam.html
especially paragraph 50.2 "Index Access Method Functions".

Now my question: can i use the (documented) Server Programming
interface within this functions (e.g. SPI_exec within ambuild)
or do i need to use other (possibly undocumented) functions.
Background: The index needs some metadata which can be created
within "normal" tables. Otherwise this information must be built
within the index.

And as an appendix to my question: Are there any (simple) examples
for custom indexes ?

greetings
uwe