PF_Cmd_SEQUENCE_SETUP called multiple times on the same plugin instance

Hello,

I am developing a PremierePro plugin and I would like to allocate an heavy structure (loading files, ...) ONCE per plugin instance.

I was thinking to use PF_Cmd_SEQUENCE_SETUP, but this is called multiple times for the same plugin instance. Even when I change one parameter in the UI, I got succesive calls to 

PF_Cmd_SEQUENCE_SETUP/PF_Cmd_SEQUENCE_FLATTEN

PF_Cmd_SEQUENCE_SETUP/PF_Cmd_SEQUENCE_SETDOWN

So the in_data->sequence_data keeps being allocated/filled/destroyed. 

Thanks in advance

David