\newsection
\subsection{First-Order Logic}
\subsubsection{Syntax}
\inlinedefinition[Signature] consists of a set of function symbols $\cF$ and a set of predicate symbols $\cP$, as well as their arities.

We write $f^k$ (or $p^k$, for predicates) to indicate that it has \textit{arity} $k \in \N$.
Constant functions have arity $0$, linear functions have arity $1$, thus, the arity of a given function (or predicate) is given by
the number of parameters to uniquely describe it, minus one.

\inlinedefinition[Term] is the terms of first-order logic is smallest set, where (with $\cV$ again a set of variables)
\begin{enumerate}
    \item $x \in \textit{Term}$ if $x \in \cV$
    \item $f^n(t_1, \ldots, t_n) \in \textit{Term}$ if $f^n \in \cF$ and $t_i \in \textit{Term}$, $\forall 1 \leq i \leq n$ (description of form of formulas)
\end{enumerate}

\inlinedefinition[Form] is the formulas of first-order logic, is the smallest set where
\begin{enumerate}
    \item $\bot \in \textit{Form}$
    \item $p^n(t_1, \ldots, t_n) \in \textit{Form}$ if $p^n \in \cP$  and $t_j \in \textit{Term}$, $\forall 1\leq j \leq n$ (description of form of predicates)
    \item $A \circ B \in \textit{Form}$ if $A \in \textit{Form}$, $B \in \textit{Form}$ and $\circ \in \{ \land, \lor, \rightarrow \}$ (i.e. formulas with logic symbols)
    \item $Qx.A \in \textit{Form}$ if $A \in \textit{Form}$, $x \in \cV$ and $Q \in \{ \forall, \exists \}$ (i.e. formulas with quantifiers)
\end{enumerate}


\paragraph{Binding}
\inlinedefinition[Bound variable] A variable that occurs in a quantifier in scope ({\color{blue} blue in example below})

\inlinedefinition[Free variable] A variable that is not bound by a quantifier in scope ({\color{red} red in example below})

\inlineexample $(q({\color{red} x}) \lor \exists {\color{blue} x}.\forall {\color{blue} y}. p(f({\color{blue} x}), {\color{red} z}) \land q({\color{red} a}))
\lor \forall {\color{blue} x}.r({\color{blue} x}, {\color{red} z}, g({\color{blue} x}))$

\inlinedefinition[$\alpha$-conversion] A \bi{bound} variable can be renamed at any time, but must preserve binding structure.


\paragraph{Precedences} 
$\neg > \land > \lor > \rightarrow$, with $>$ a total order of precedences.
Quantifiers extend as far right as possible, bounded by the end of line or a going out of scope by closing parenthesis.
