If segment exists do nothing if not add

Does anyone know the correct syntax for this?

I want to check to see if a PD1 segment exists. If it does then if PD1.3.1 is populated, leave it alone and send the message.

If PD1.3.1 is blank then copy PV1.3.1 to PD1.3.1

if PD1 does not exist then I want to add PD1 after PID and then copy PV1.3.1 to PD1.3.1

If segment exists do nothing if not add