**O**nce again working on my slides for the AMSI Lecture 2012 tour, it took me a while to get the following LaTeX code (about the family reunion puzzle) to work:

\begin{frame}[fragile,label=notleM2] \slidetitle{A family meeting} \begin{block}{Random switch of couples} \only<1>{ \begin{itemize} \item Pick two couples [among the 20 couples] at random with probabilities proportional to the number of other couples they have not seen\\ {\verb+prob=apply(meet(T)==0,1,sum)+} \item switch their respective position during one of the 6 courses \item accept the switch with Metropolis--Hsatings probability\\ {\verb#log(runif(1))<(penalty(old)-penalty(new))/gamma#} \end{itemize} } \only<2>{ \begin{verbatim} for (t in 1:N){ prop=sample(1:20,2,prob=apply(meet(T)==0,1,sum)) cour=sample(1:6,1) Tp=T Tp[prop[1],cour]=T[prop[2],cour] Tp[prop[2],cour]=T[prop[1],cour] penatp=penalty(meet(Tp)) if (log(runif(1))<(penat-penatp)/gamma){ T=Tp penat=penatp} } \end{verbatim} } \end{block} \end{frame}

since I was getting error messages of the form

[86] (./simulation.38.vrb) [87] (./simulation.39.vrb !Illegal parameter number in definition of \beamer@doifinframe. l l.12 } ?

Using two frames in a row instead of the “only<2>” version did not help…

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