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+\section{Cardinality}
+
+\import{set.tex}
+\import{ordinal.tex}
+\import{function.tex}
+
+\begin{definition}\label{finite}
+    $X$ is finite iff there exists a natural number $k$ such that
+        there exists a bijection from $k$ to $X$.
+\end{definition}
+
+\begin{abbreviation}\label{infinite}
+    $X$ is infinite iff $X$ is not finite.
+\end{abbreviation}