Suppose I am going to create a Purchase Order (PO), OpenERP will auto generate the sequence of the PO according to the settings. Can I change the way my PO sequence runs as the following conditions:

If the PO I create has tax value, the PO number is PO / 00001 as its normally would,

but if it does not has tax value, the PO number change to something likes PX / 00001.

Could anyone suggests how to do this implementation? Either via web interface or dive into the code.

Thanks you very much