Let $\mathfrak{A}$ be an uncountable collection of Borel sets in $\mathbb{R}^d$ such that for any $A,B\in\mathfrak{A}$, either $A\subset B$ or $A\supset B$. Then is it necessarily true that the union $\cup_{A\in\mathfrak{A}}A$ is Borel measurable?
Union of ordered Borel sets
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