Hi Lingjie,
Using "--filter lookahead" (or 'filter="lookahead"' if you're using RBi)
would be a start. If you don't specify lookahead_transition and
lookahead_observation the transition and observations densities from the
full model are used for the lookahead part.
If you want to using different transition and observation densities you
can use the lookahead_transition and lookahead_observation blocks which
use the same syntax as the transition and observation blocks, which you
can find in the examples.
Cheers,
Seb.