I want to keep a subset @b of @a if and only if the elements of @b are adjacent in @a.

This is a equivalent to saying that @b is a slice of @a - at least that what it seems like from your example. As grinder pointed out, to generate slices you just need to pick a starting point and a length.

However, I can think of another possible interpretation of 'adjacent' which allows the sequences BA and CBA, etc.