I've got :
set security policies from-zone trust to-zone untrust policy trust-to-untrust then log session-init
set security policies from-zone trust to-zone untrust policy trust-to-untrust then log session-close
and
set security policies from-zone untrust to-zone trust policy untrust-to-trust then log session-init
set security policies from-zone untrust to-zone trust policy untrust-to-trust then log session-close
I hope that init -> close is the same close -> init regardging position in sequence