add a citation in the intro

paper-new/intro.tex

@@ -171,7 +171,7 @@ corresponding technical reports having 50+ pages of proofs.
 \end{comment}
 
 An alternative approach, which we investigate in this paper, is provided by
-\emph{synthetic guarded domain theory}.
+\emph{synthetic guarded domain theory}\cite{birkedal-mogelberg-schwinghammer-stovring2011}.
 The techniques of synthetic guarded domain theory allow us to internalize the
 step-indexed reasoning normally required in logical relations proofs of graduality,
 ultimately allowing us to specify the logical relation in a manner that looks nearly