Display Filter - Byte Offset Notation

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Using version 0.99.0, and am struggling to create a simple display
filter using byte offset notation. I want to simply capture traffic
where the first two bytes of the source address are 68.154. Shouldn't
this filter be as simple as ip[12:2]==68 154? I've tried lots of
different permutations, but can't get any to work. I have created the
same offset filter in another product, Network Instruments Observer, and
I get the results I would expect.