\documentclass[dvipsnames,14pt,t]{beamer}
\usepackage{../slides}
\usepackage{../langs}
\usepackage{../graphics}
\usepackage{../grammar}

% beamer stuff 
\renewcommand{\slidecaption}{APP 09, King's College London}
\newcommand{\bl}[1]{\textcolor{blue}{#1}}

\begin{document}

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\begin{frame}[t]
\frametitle{%
  \begin{tabular}{@ {}c@ {}}
  \\
  \LARGE Access Control and \\[-3mm] 
  \LARGE Privacy Policies (9)\\[-6mm] 
  \end{tabular}}\bigskip\bigskip\bigskip

  \normalsize
  \begin{center}
  \begin{tabular}{ll}
  Email:  & christian.urban at kcl.ac.uk\\
  Office: & S1.27 (1st floor Strand Building)\\
  Slides: & KEATS (also homework is there)\\
  \end{tabular}
  \end{center}

\end{frame}
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
  \begin{frame}[c]

  \begin{center}
  \includegraphics[scale=0.6]{../pics/bridge-limits.png}
  \end{center}

  \end{frame}
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
  \begin{frame}[c]
  \frametitle{Old-Fashioned Eng.~vs.~CS}
  
  \begin{center}
  \begin{tabular}{@{}cc@{}}
  \begin{tabular}{@{}p{5.2cm}} 
  \includegraphics[scale=0.058]{../pics/towerbridge.jpg}\\ 
  {\bf bridges}: \\
  \raggedright\small
  engineers can ``look'' at a bridge and have a pretty good
  intuition about whether it will hold up or not\\ 
  (redundancy; predictive theory)
  \end{tabular} &
  \begin{tabular}{p{5cm}} 
  \includegraphics[scale=0.265]{../pics/code.jpg}\\
  \raggedright
  {\bf code}: \\
  \raggedright\small
  programmers have very little intuition about their code; 
  often it is too expensive to have redundancy;
  not ``continuous'' 
  \end{tabular}
  \end{tabular}
  \end{center}

  \end{frame}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%


%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
  \begin{frame}[c]
  \frametitle{Dijkstra on Testing}
  
  \begin{bubble}[10cm]
  ``Program testing can be a very effective way to show the
  presence of bugs, but it is hopelessly inadequate for showing
  their absence.''
  \end{bubble}\bigskip
  
  unfortunately attackers exploit bugs (Satan's computer vs 
  Murphy's)

  \vfill
  \small
  Dijkstra: shortest path algorithm, 
  dining philosophers problem,
  semaphores
  \end{frame}
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\begin{frame}[c]
\frametitle{\Large Proving Programs to be Correct}

\begin{bubble}[10cm]
\small
{\bf Theorem:} There are infinitely many prime 
numbers.\medskip\\

{\bf Proof} \ldots\\
\end{bubble}\bigskip


similarly\bigskip

\begin{bubble}[10cm]
\small
{\bf Theorem:} The program is doing what 
it is supposed to be doing.\medskip

{\bf Long, long proof} \ldots\\
\end{bubble}\bigskip\medskip

\small This can be a gigantic proof. The only hope is to have
help from the computer. `Program' is here to be understood to be
quite general (protocol, OS,\ldots).

\end{frame}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%  

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
  \begin{frame}[c]
  \frametitle{\Large{}Mars Pathfinder Mission 1997}

  \begin{center}
  \includegraphics[scale=0.15]{../pics/marspath1.png}
  \includegraphics[scale=0.16]{../pics/marspath3.png}
  \includegraphics[scale=0.3]{../pics/marsrover.png}
  \end{center}
  
  \begin{itemize}
  \item despite NASA's famous testing procedures, the lander crashed frequently on Mars
  \item a scheduling algorithm was not used in the OS
  \end{itemize}

  \end{frame}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%  

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
  \begin{frame}[c]
  

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  \end{tabular}
  \end{textblock}

  \begin{textblock}{0}(2.5,13)%
  \small
  \onslide<3->{
  \begin{bubble}[8cm]%
  Scheduling: You want to avoid that a high 
  priority process is staved indefinitely.
  \end{bubble}}
  \end{textblock}

  \end{frame}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%  
  
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
  \begin{frame}[c]
  \frametitle{\Large Priority Inheritance Scheduling}

  \begin{itemize}
  \item Let a low priority process $L$ temporarily inherit 
    the high priority of $H$ until $L$ leaves the critical 
    section unlocking the resource.\bigskip
  \item Once the resource is unlocked $L$ returns to its original 
    priority level.
  \end{itemize}

  \end{frame}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%  
  
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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    \draw[fill, blue!50] (4.5,0) rectangle (5,0.6);
    \draw[very thick, blue!50] (5,0) rectangle (7,0.6);
    \node at (1.5,0.9) {\small $A_L$};
    \node at (2.0,0.9) {\small $B_L$};
    \node at (3.5,3.9) {\small $A_R$};
    \node at (6.7,0.9) {\small $B_R$};
    \draw[very thick,blue!100] (1.5,0) -- (1.5,0.6);
    \draw[very thick,blue!100] (2.0,0) -- (2.0,0.6);
    \draw[very thick,blue!100] (3.5,3) -- (3.5,3.6);
    \draw[very thick,blue!100] (6.7,0) -- (6.7,0.6);
    \draw[fill,blue!100] (3.5,3) rectangle (4.5,3.6);
    \node at (4,3.3) {\small $A$};
    \draw[very thick,blue!100] (7,3) rectangle (8,3.6);
    \node at (7.5,3.3) {\small $B$};  
    \draw[red, fill] (5,1.5) rectangle (6,2.1);
    \draw[red, fill] (6.05,1.5) rectangle (7,2.1);
    \draw[blue!50, ->, line width = 2mm] (7.1,0.4) -- (8, 0.4);
    \draw[blue!50, ->, line width = 2mm] (7.1,4) -- (8,4);
  }

  \draw[very thick,->](0,0) -- (8,0);
  \node [anchor=base] at (8, -0.3) {\scriptsize time};
  \node [anchor=base] at (0, -0.3) {\scriptsize 0};
  \node [anchor=base] at (-1.2, 0.2) {\small low priority};
  \only<2->{\node [anchor=base] at (-1.2, 3.2) {\small high priority};}
  \only<10->{\node [anchor=base] at (-1.5, 1.7) {\small medium pr.};}

  \end{tikzpicture}
  \end{tabular}
  \end{textblock}
  
  \begin{textblock}{0}(2.5,13)%
  \small
  \onslide<11>{
  \begin{bubble}[8cm]%
  Scheduling: You want to avoid that a high 
  priority process is staved indefinitely.
  \end{bubble}}
  \end{textblock}


  \end{frame}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%  
  
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
  \begin{frame}[c]
  \frametitle{\Large Priority Inheritance Scheduling}

  \begin{itemize}
  \item Let a low priority process $L$ temporarily inherit 
    the high priority of $H$ until $L$ leaves the critical 
    section unlocking the resource.\bigskip
  \item Once the resource is unlocked $L$ returns to its original 
    priority level. \alert{\bf BOGUS}\pause\bigskip
  \item \ldots $L$ needs to switch to the highest 
    \alert{remaining} priority of the threads that it blocks.
  \end{itemize}\bigskip

  \small this error is already known since around 1999

  \end{frame}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%  

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
  \begin{frame}[c]

  \begin{center}
  \includegraphics[scale=0.25]{../pics/p3.jpg}
  \end{center}

   \begin{itemize}
  \item by Rajkumar, 1991
  \item \it ``it resumes the priority it had at the point of entry into the critical 
  section''
  \end{itemize}

  \end{frame}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%  
     

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
  \begin{frame}[c]

  \begin{center}
  \includegraphics[scale=0.25]{../pics/p2.jpg}
  \end{center}

   \begin{itemize}
  \item by Jane Liu, 2000
  \item {\it ``The job $J_l$ executes at its inherited 
    priority until it releases $R$; at that time, the 
    priority of $J_l$ returns to its priority 
    at the time when it acquires the resource $R$.''}\medskip
  \item \small gives pseudo code and totally bogus data structures  
  \item \small interesting part ``{\it left as an exercise}''
  \end{itemize}

  \end{frame}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%  
     
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
  \begin{frame}[c]

  \begin{center}
  \includegraphics[scale=0.15]{../pics/p1.pdf}
  \end{center}

  \begin{itemize}
  \item by Laplante and Ovaska, 2011 (\$113.76)
  \item \it ``when $[$the task$]$ exits the critical section that
        caused the block, it reverts to the priority it had
        when it entered that section'' 
  \end{itemize}

  \end{frame}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%  
  
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
  \begin{frame}[c]
  \frametitle{Priority Scheduling}

  \begin{itemize}
  \item a scheduling algorithm that is widely used in real-time operating systems
  \item has been ``proved'' correct by hand in a paper in 1983
  \item but this algorithm turned out to be incorrect, despite its ``proof''\bigskip\pause
  
  \item we corrected the algorithm and then {\bf really} proved that it is correct	
  \item we implemented this algorithm in a small OS called PINTOS (used for teaching at Stanford)
  \item our implementation was much more efficient than their reference implementation
  \end{itemize}

  \end{frame}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%  
   
   
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
  \begin{frame}[c]
  \frametitle{Design of AC-Policies}

  \Large
  \begin{quote}
  ''what you specify is what you get but not necessarily what you want\ldots''
  \end{quote}\bigskip\bigskip\bigskip
  
  \normalsize main work by Chunhan Wu (PhD-student)

  \end{frame}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%     

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
  \begin{frame}[c]
  \frametitle{Testing Policies}

  \begin{center}
  \begin{tikzpicture}[scale=1]
  %\draw[black!10,step=2mm] (-5,-3) grid (5,4);
  %\draw[black!10,thick,step=10mm] (-5,-3) grid (5,4);
  \draw[white,thick,step=10mm] (-5,-3) grid (5,4);

  \draw [black!20, line width=1mm] (0,0) circle (1cm);
  \draw [line width=1mm] (0,0) circle (3cm);
  \node [black!20] at (0,0) {\begin{tabular}{c}core\\[-1mm] system\end{tabular}};
  
  \draw [red, line width=2mm, <-] (-2.1,2.1) -- (-3.5,3.2);
  \node at (-3.5,3.6) {your system};
  \node at (-4.8,0) {\Large policy $+$};

  
  \only<2>{
  \draw [black, fill=red, line width=0.5mm] (2,1) circle (3mm);
  \draw [red, line width=2mm, <-] (2.3,1.2) -- (3.5,2);
  \node at (3.8,2.6) {\begin{tabular}{l}a seed\\[-1mm] \footnotesize virus, Trojan\end{tabular}};}

  \only<3>{
  \draw [black, fill=red, line width=0.5mm] (2,1) circle (7mm);
  \node[white] at (2,1) {\small tainted};}

  \only<4>{
  \begin{scope}
  \draw [clip] (0,0) circle (2.955cm);
  \draw [black, fill=red, line width=0.5mm] (2,1) circle (9mm);
  \node[white] at (2,1) {\small tainted};
  \end{scope}}

  \only<5->{
  \begin{scope}
  \draw [clip] (0,0) circle (2.955cm);
  \draw [black, fill=red, line width=0.5mm] (2,1) circle (13mm);
  \node[white] at (2,1) {\small tainted};
  \end{scope}}

  \only<6->{
  \draw[fill=white, line width=1mm] (1.265,2.665) arc (-35:183:5mm);
  \draw[fill=white, line width=1mm] (1.25,3.25) arc (-35:183:3mm);
  \node[black, rotate=10] at (1.9,3.6) {\LARGE\ldots};
  }

  \end{tikzpicture}
  \end{center}

  \end{frame}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% 

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
  \begin{frame}[c]
  \frametitle{A Sound and Complete Test}

  \begin{itemize}
  \item working purely in the \emph{dynamic world} does not work -\!-\!- infinite state space\bigskip

  \item working purely on \emph{static} policies also does not\\ work -\!-\!- because of over approximation
  \smallskip
  \begin{itemize}
  \item suppose a tainted file has type \emph{bin} and
  \item there is a role \bl{$r$} which can both read and write \emph{bin}-files\pause
  \item then we would conclude that this tainted file can spread\medskip\pause
  \item but if there is no process with role \bl{$r$} and it will never been
  created, then the file actually does not spread
  \end{itemize}\bigskip\pause

  \item \alert{our solution:} take a middle ground and record precisely the information
  of the initial state, but be less precise about every newly created object.
  \end{itemize}

  \end{frame}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%     

\begin{frame}[c]
\frametitle{Big Proofs in CS}

Formal proofs in CS sound like science fiction? Completely irrelevant!
Lecturer gone mad?\pause

\begin{itemize}
\item in 2008, verification of a small C-compiler
\begin{itemize}
\item ``if my input program has a certain behaviour, then the compiled machine code has the same behaviour''
\item is as good as \texttt{gcc -O1}, but much less buggy 
\end{itemize}
\medskip 
\item in 2010, verification of a micro-kernel operating system (approximately 8700 loc)
\begin{itemize}
\item 200k loc of proof
\item 25 - 30 person years
\item found 160 bugs in the C code (144 by the proof)
\end{itemize}
\end{itemize}

\end{frame}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\begin{frame}[c]
\frametitle{Goal}

Remember the Bridges example?

\begin{itemize}
\item Can we look at our programs and somehow ensure
they are secure/bug free/correct?\pause\bigskip

\item Very hard: Anything interesting about programs is equivalent
to halting problem, which is undecidable.\pause\bigskip

\item \alert{Solution:} We avoid this ``minor'' obstacle by
      being as close as possible of deciding the halting
      problem, without actually deciding the halting problem.
      $\quad\Rightarrow$ static analysis
\end{itemize}

\end{frame}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% 

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
  \begin{frame}[c]
  \frametitle{What is Static Analysis?}

  \begin{center}
  \includegraphics[scale=0.4]{../pics/state.png}
  \end{center}
  
  \begin{itemize}
  \item depending on some initial input, a program 
  (behaviour) will ``develop'' over time.
  \end{itemize}

  \end{frame}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
  \begin{frame}[c]
  \frametitle{What is Static Analysis?}

  \begin{center}
  \includegraphics[scale=0.4]{../pics/state2.png}
  \end{center}

  \end{frame}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
  \begin{frame}[c]
  \frametitle{What is Static Analysis?}

  \begin{center}
  \includegraphics[scale=0.4]{../pics/state3.jpg}
  \end{center}

  \end{frame}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
  \begin{frame}[c]
  \frametitle{What is Static Analysis?}

  \begin{center}
  \includegraphics[scale=0.4]{../pics/state4.jpg}
  \end{center}

  \begin{itemize}
  \item to be avoided
  \end{itemize}

  \end{frame}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
  \begin{frame}[c]
  \frametitle{What is Static Analysis?}

  \begin{center}
  \includegraphics[scale=0.4]{../pics/state5.png}
  \end{center}
  
  \begin{itemize}
  \item this needs more work
  \end{itemize}

  \end{frame}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
  \begin{frame}[c]
  \frametitle{What is Static Analysis?}

  \begin{center}
  \includegraphics[scale=0.4]{../pics/state6.png}
  \end{center}

  \end{frame}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\begin{frame}[fragile]
\frametitle{\large Concrete Example: Sign-Analysis}
\mbox{}\\[-20mm]\mbox{}

\bl{\begin{plstx}[rhs style=,one per line]
: \meta{Exp} ::= \meta{Exp} + \meta{Exp}
         | \meta{Exp} * \meta{Exp}
         | \meta{Exp} = \meta{Exp} 
         | \meta{num}
         | \meta{var}\\
: \meta{Stmt} ::= \meta{label} :
         | \meta{var} := \meta{Exp}
         | jump? \meta{Exp} \meta{label}
         | goto \meta{label}\\
: \meta{Prog} ::= \meta{Stmt} \ldots\\
\end{plstx}}

\end{frame}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\begin{frame}[fragile]
\frametitle{\large Concrete Example: Sign-Analysis}
\mbox{}\\[-20mm]\mbox{}

\bl{\begin{plstx}[rhs style=,one per line]
: \meta{Exp} ::= \meta{Exp} + \meta{Exp}
         | \meta{Exp} * \meta{Exp}
         | \meta{Exp} = \meta{Exp} 
         | \meta{num}
         | \meta{var}\\
: \meta{Stmt} ::= \meta{label} :
         | \meta{var} := \meta{Exp}
         | jump? \meta{Exp} \meta{label}
         | goto \meta{label}\\
: \meta{Prog} ::= \meta{Stmt} \ldots\\
\end{plstx}}

\begin{textblock}{0}(7.8,2.5)
\small
\begin{bubble}[5.6cm]
\begin{lstlisting}[numbers=none,
                   basicstyle=\ttfamily,
                   language={},xleftmargin=3]
      a := 1
      n := 5 
top:  jump? n = 0 done 
      a := a * n 
      n := n + -1 
      goto top 
done:
\end{lstlisting}
\end{bubble}
\end{textblock}

\end{frame}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\begin{frame}[fragile]
\frametitle{\large Concrete Example: Sign-Analysis}
\mbox{}\\[-20mm]\mbox{}

\bl{\begin{plstx}[rhs style=,one per line]
: \meta{Exp} ::= \meta{Exp} + \meta{Exp}
         | \meta{Exp} * \meta{Exp}
         | \meta{Exp} = \meta{Exp} 
         | \meta{num}
         | \meta{var}\\
: \meta{Stmt} ::= \meta{label} :
         | \meta{var} := \meta{Exp}
         | jump? \meta{Exp} \meta{label}
         | goto \meta{label}\\
: \meta{Prog} ::= \meta{Stmt} \ldots\\
\end{plstx}}

\begin{textblock}{0}(7.8,3.5)
\small
\begin{bubble}[5.6cm]
\begin{lstlisting}[numbers=none,
                   basicstyle=\ttfamily,
                   language={},xleftmargin=3]
      n := 6
      m1 := 0
      m2 := 1
top:  jump? n = 0 done
      tmp := m2
      m2 := m1 + m2
      m1 := tmp
      n := n + -1
      goto top
done:
\end{lstlisting}
\end{bubble}
\end{textblock}

\end{frame}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%



\end{document}

%         n := 5 
% top:    jump? n = 0 done 
%         a := a * n 
%         n := n + -1 
%         goto top 
% done:


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