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\begin{frame}
  \frametitle{Relationship Sets and Role Names}

  \begin{goal}{}
    The relationship set connections can be annotated with \emph{role indicators}.
  \end{goal}
  \medskip

  \begin{exampleblock}{}
  \begin{center}
  \begin{tikzpicture}[every edge/.style={link},node distance=35mm,>=triangle 45,inner sep=.5mm]
    \begin{scope}
      \node[relationship] (R) {buys-from};
      \node (l) [left of=R,entity] {customer}; \draw (R) -- node[above,pos=.5] {buyer} (l); 
      \node (r) [right of=R,entity] {shop}; \draw (R) -- node[above,pos=.5] {seller} (r);
    \end{scope}
  \end{tikzpicture}
  \end{center}
  \end{exampleblock}
  \begin{itemize}
    \item role indicators improve readability
  \end{itemize}
\end{frame}