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\begin{signature}\label{example_cons}
$\cons{y}{X}$ is a set.
\end{signature}
\begin{axiom}\label{cons}
$x\in \cons{y}{X}$ iff $x = y$ or $x\in X$.
\end{axiom}
\begin{definition}\label{unit}
$\unit = \{\emptyset\}$.
\end{definition}
\begin{proposition}\label{emptyset_in_unit}
$\emptyset\in\unit$.
\end{proposition}
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