[Dovecot] dovecot-virtual folder configuration

[Pierre]

Hi all,

Long story short, I'd like to configure a folder containing:

INBOX
(some other folders)

-> unseen emails from these folders + all other emails (also seen) from 
the same thread
-> seen emails from these folders, but only the recent ones + all other 
emails from the same thread


I tried:
#    inthread refs (or (or recent unseen) recent )
     inthread refs unseen
# inthread refs (or unseen (  recent seen) )
#    inthread refs  recent unseen
# inthread refs unseen
#inthread refs recent (or unseen )
# inthread refs (recent seen (or unseen ))
# (inthread refs  (or unseen) recent seen)
#( inthread refs (or (or recent unseen) flagged) )

without success.

Where could I find proper,understandable doc for this?
Could anybody help me on configuring such rule(s) ?

Thanks a lot in adavance,

Pierre