indexing tables using my owns functions

Hi every body

Recently i wrote my own function into postgesql... and it works perfecly!!,
now i would like use it to index a table like this guy do at the link (
http://www.faqs.org/docs/ppbook/r24254.htm)

but i get the following error

mhc2db=> create index h2iab on precalc (idr(p1,p4,p6,p7,p9,'H-2*IAb'));
ERROR: functions in index expression must be marked IMMUTABLE
mhc2db=>

in this indexation p1,p4,p6,p7,p9 are variables that depends on the field.

could anyone help me?

--
Pau Marc Muñoz Torres

Laboratori de Biologia Computacional
Institut de Biotecnologia i Biomedicina Vicent
Villar
Universitat Autonoma de Barcelona
E-08193 Bellaterra (Barcelona)

telèfon: 93 5812807
Email : paumarc(dot)munoz(at)bioinf(dot)uab(dot)cat