temporary link to prophecy MR, until we can reference a paper instead

docs/language.tex

 \section{Language}
 \label{sec:language}
 
-A \emph{language} $\Lang$ consists of a set \Expr{} of \emph{expressions} (metavariable $\expr$), a set \Val{} of \emph{values} (metavariable $\val$), a set $\Obs$ of \emph{observations} (or ``observable events'') and a set $\State$ of \emph{states} (metavariable $\state$) such that
+A \emph{language} $\Lang$ consists of a set \Expr{} of \emph{expressions} (metavariable $\expr$), a set \Val{} of \emph{values} (metavariable $\val$), a set $\Obs$ of \emph{observations}\footnote{See \url{https://gitlab.mpi-sws.org/iris/iris/merge_requests/173} for how observations are useful to encode prophecy variables.} (or ``observable events'') and a set $\State$ of \emph{states} (metavariable $\state$) such that
 \begin{itemize}[itemsep=0pt]
 \item There exist functions $\ofval : \Val \to \Expr$ and $\toval : \Expr \pfn \Val$ (notice the latter is partial), such that
 \begin{mathpar}