Functional Indices

Hello,
I want to create functional index for a C function that i have written.

the function looks like....
float8
fx(float8,float8) {
..
}

I also have a table T(x1,x2) where x1 and x2 are attributes of type
float8.

The function takes x1,x2 as inputs and outputs fx(x1,x2) ...

How do i create an index on fx(x1,x2) ????

I tried the documnetation but there are no good examples for functional
indices....

Any help is would be really nice...

My e-mail :- nilesh(at)vt(dot)edu

Thanks in advance
Nilesh.