function module requirement

Hi..

I am an XI guy .. actually i need an function module where that should function the increment and return that increment value..

eg: when it initially executes it shuld be 001 and when ever it is called it should be incremented by 1 i.e 002 and return that value,

here i will import this rfc to xi and will be used for the sequence operation.

may i know the procedure or if any one helps me out it will be really helpful for the process..

regards

sai