Hi Rick,
The rshuffle function will output a sequence or randomly picked, evenly spaced trials of each label, but the order of the labels is indeed random. So for rshuffle("A","filler","B") you can get six sequence types: [A,filler,B,A,filler,B,etc.], [A,B,filler,A,B,filler,etc.], [filler,B,A,filler,B,A,etc.], [filler,A,B,filler,A,B,etc.], [B,A,filler,B,A,filler,etc.] or [B,filler,A,B,filler,A,etc.].
What do the (i)s refer to in your message?
In any case, it sounds like you could make use of the custom subsequence function defined on this thread. Then you can do this: Sequence( subsequence(randomize("A"),randomize("filler"),randomize("B")) )
Let me know if you have questions
Jeremy