Tvheadend

Hello,

I have some questions according these sequence: 

When starting a stream TvHeadend is sending an SETUP (note no: "&pids=" statement):
    SETUP rtsp://192.168.0.110/?fe=5&freq=610&sr=6900&msys=dvbc&mtype=256qam&fec=35

After this the pids are added by sending PLAY:
    PLAY rtsp://192.168.0.110/stream=0?addpids=0,1,16,17,18,2000,2001,2011,2012,2101,2102,2103,8187
    Session: 0265641043

Changing to another channel the following is send (note no: "&pids=" statement again):
    SETUP rtsp://192.168.0.110/stream=0?fe=5&freq=610&sr=6900&msys=dvbc&mtype=256qam&fec=35
    Session: 0265641043

After this the pids are added by sending PLAY:
    PLAY rtsp://192.168.0.110/stream=0?addpids=0,1,16,17,18,2300,2301,2311,2312,2401,2402,2403,8187
    Session: 0265641043

My questions are these:
    - Why are the old pids not deleted (with "pids=" statement) ?
    - Is this according to the specs?

My understanding of the specs is this:
    - An "pids=" statement will/must replace all previouse list
    - That when changing channels at least the "pids=" statement should be
      send to prevent hanging pids

The specs also state:
    - An in-session SETUP without a query in the RTSP URI shall not modify the list of PIDs to be filtered
      BUT DOES THIS MEAN when there is, the pid list must be cleared as well?
      AND THEREFORE THIS IS correct what TvHeadend is doing?

Can someone clear up this issue for me?