If $D = C$, trivial. For $D \neq C$:
$D \isin C \equiv D \isin A \equiv D \le A \equiv D \le C$. $\qed$
-\section{Test more symbols}
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-$ C \haspatch \p $
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-$ C \nothaspatch \p $
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-$ \p \patchisin C $
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-$ \p \notpatchisin C $
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-$ \{ B \} \areparents C $
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\end{document}