Hi Nianpo,
So, your Sequence line is Sequence( randomize(startsWith("Long") , randomize(startsWith("Short") ))), which corresponds to what I originally included in my post; what you included in your post, Sequence( rshuffle(startsWith("Short") , rshuffle(startsWith("Long") ))) does include “Short” trials
There is a typo in the former, which I since corrected by editing my post, where the closing parenthesis of the first randomize only comes after the second randomize, effectively ignoring that one and thus just including the randomized “Long” trials. The line you want to use is Sequence( randomize(startsWith("Long")) , randomize(startsWith("Short")) )
Jeremy