Need a Crystal Report formula to concatenate strings during reading records

I have a report that takes partnumbers and cross references them with other partnumbers.

I have this information in the details

Partnumber XRef I need this as the string

A B B

A C B,C

A D B,C,D

And then start the a new string when it gets to a new partnumber. Does anyone know how to do this?

Thanks

siboyd