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The lemma:

Any uncountable set $S$ of finite sets has an uncountable subset $\Delta \subseteq S$ and an $x$ such that $\forall a,b \in \Delta$, if $a \neq b$ then $a \cap b = x$. $\Delta$ is called a $\Delta$-system.

I've seen this lemma used in independence proofs, such as the famous result that the negation of the continuum hypothesis is consistent with ZFC, but it seems like it would be useful in other fields as well. Does anyone have any examples with this lemma outside pure set theory?

Any uncountable set $S$ of finite sets has an uncountable subset $\Delta \subseteq S$ and a member an $x \in S$ x$such that$\forall a,b \in \Delta$, if$a \neq b$then$a \cap b = x$.$\Delta$is called a$\Delta\$-system.