= Unbounded finite shattering without an infinite universal trace
{title2=$\mathcal F=\bigcup_{j\geq1}\mathcal P(B_j)$}
Partition an infinite countable set into disjoint finite blocks of unbounded size. The union of their power sets consists entirely of finite sets and shatters each block, so it has unbounded finite <VC dimension>. Every infinite set meets two different blocks. The pair formed by one point from each can never occur as a trace, since each family member stays in one block. Thus no infinite set has traces realizing all its finite subsets.
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