Sorry to bother you again!
I had tried:
Sequence( rshuffle("A","filler","B") );
but it seems that the trials order is random, instead of A(i) filler(i) B(i), but A B filler (all the way) or Filler B A (all the way) or other kind of orders,
I tried to debug this for a while (to exclude other possibility and keep only A filler B)and found nothing wrong with this expression, could you offer me some suggestions like what I should check to resolve this problem? (If my understanding is not wrong, rshufulle() means keeping the order of A-filler-B, but randomly select elements from materials labelled as A, filler and B to the experiment)