Housekeeping.

src/logic/index.tex

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+\part{Mathematical Logic}
+\label{part:logic}
+
+This section is plagiarised from 

src/logic/tp/index.tex

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+\chapter{Truth and Proof}
+\label{chap:truth-and-proof}
+
+\input{./lang.tex}
+

src/logic/tp/lang.tex

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+\section{Languages}
+\label{section:languages}
+
+\begin{definition}[Language]
+\label{definition:language}
+    A \textbf{language} is a triplet of symbol sets $(\mathcal{F}, \mathcal{R}, \mathcal{C})$ and assignments $n_{\cdot}: \mathcal{F} \to \natp$ and $n_{\cdot}: \mathcal{R} \to \natp$, where:
+    \begin{enumerate}
+        \item For each $f \in \mathcal{F}$, $f$ is a function symbol in $n_{f}$ variables.
+        \item For each $R \in \mathcal{R}$, $R$ is a $n_R$-ary relation symbol. 
+        \item For each $C \in \mathcal{C}$, $C$ is a constant symbol.
+    \end{enumerate}
+\end{definition}
+
+
+