A \emph{language}$\Lang$ consists of a set \Expr{} of \emph{expressions} (metavariable $\expr$), a set \Val{} of \emph{values} (metavariable $\val$), and a nonempty set \State of \emph{states} (metavariable $\state$) such that

\begin{itemize}

\begin{itemize}[itemsep=0pt]

\item There exist functions $\ofval : \Val\to\Expr$ and $\toval : \Expr\pfn\Val$ (notice the latter is partial), such that