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