A \emph{language}$\Lang$ consists of a set \textdom{Expr} of \emph{expressions} (metavariable $\expr$), a set \textdom{Val} of \emph{values} (metavariable $\val$), and a set \textdom{State} of \emph{states} (metvariable $\state$) such that
\begin{itemize}
\item There exist functions $\ofval : \textdom{Val}\to\textdom{Expr}$ and $\toval : \textdom{Expr}\pfn\textdom{val}$ (notice the latter is partial), such that
\item There exists a \emph{primitive reduction relation}\[(-,-\step-,-,-)\subseteq\textdom{Expr}\times\textdom{State}\times\textdom{Expr}\times\textdom{State}\times(\cup_n \textdom{Expr}^n)\]
A reduction $\expr_1, \state_1\step\expr_2, \state_2, \overline\expr$ indicates that, when $\expr_1$ reduces to $\expr_2$, the new threads in the list $\overline\expr$ is forked off.
We will write $\expr_1, \state_1\step\expr_2, \state_2$ for $\expr_1, \state_1\step\expr_2, \state_2, ()$, \ie when no threads are forked off. \\
\item All values are stuck:
\[\expr, \_\step\_, \_, \_\Ra\toval(\expr)=\bot\]
\end{itemize}
\begin{defn}
An expression $\expr$ and state $\state$ are \emph{reducible} (written $\red(\expr, \state)$) if
A \emph{language}$\Lang$ consists of a set \textdom{Expr} of \emph{expressions} (metavariable $\expr$), a set \textdom{Val} of \emph{values} (metavariable $\val$), and a set \textdom{State} of \emph{states} (metvariable $\state$) such that
\begin{itemize}
\item There exist functions $\ofval : \textdom{Val}\to\textdom{Expr}$ and $\toval : \textdom{Expr}\pfn\textdom{val}$ (notice the latter is partial), such that
\item There exists a \emph{primitive reduction relation}\[(-,-\step-,-,-)\subseteq\textdom{Expr}\times\textdom{State}\times\textdom{Expr}\times\textdom{State}\times(\cup_n \textdom{Expr}^n)\]
A reduction $\expr_1, \state_1\step\expr_2, \state_2, \overline\expr$ indicates that, when $\expr_1$ reduces to $\expr_2$, the new threads in the list $\overline\expr$ is forked off.
We will write $\expr_1, \state_1\step\expr_2, \state_2$ for $\expr_1, \state_1\step\expr_2, \state_2, ()$, \ie when no threads are forked off. \\
\item All values are stuck:
\[\expr, \_\step\_, \_, \_\Ra\toval(\expr)=\bot\]
\end{itemize}
\begin{defn}
An expression $\expr$ and state $\state$ are \emph{reducible} (written $\red(\expr, \state)$) if