Indices/Access Methods (not B+ Trees)

Hi, I'm doing a project for a DB class I'm taking. I've been looking into disk-based index methods for mulit-dimensional data. I've heard that Informix's recent DBMSs ship with R-tree as one of the access methods. Other versions of the R-tree are apparently implemented in various DataBlades. Has anyone worked with R-trees or any other non-hash, non-B-tree access methods with Informix's DBMS? Have any of you implemented your own access method with Informix's Universal Data Option? I'd love to hear any comments. (Speed, ease of use, effort involved, etc.) Feel free to e-mail me at tbryan@arlut.utexas.edu ---Tom