\begin{frame}
  \frametitle{Degree of a Relationship Set}
  
  \begin{center}
    \scalebox{.9}{
    \begin{tikzpicture}[every edge/.style={link}]
      \node[entity] (A) {A};
      \node[entity,right of=A, node distance=6cm] (B) {B};
      \node[entity,right of=A, node distance=3cm,yshift=2cm] (C) {C};

      \node[relationship,scale=.9,minimum size=10mm] at ($(A)!.5!(B)$) {R} edge (A) edge (B) edge (C);
    \end{tikzpicture}}
  \end{center}
  
  \begin{goal}{}
    Non-binary relationship sets can be represented using binary ones
    by creating an artificial entity set.
  \end{goal}
  
  \begin{center}
    \scalebox{.9}{
    \begin{tikzpicture}[every edge/.style={link}]
      \node[entity] (A) {A};
      \node[entity,right of=A, node distance=6cm] (B) {B};
      \node[entity,right of=A, node distance=3cm,yshift=2.5cm] (C) {C};

      \node[entity,minimum size=10mm] at ($(A)!.5!(B)$) (E) {E};
      \node[relationship,scale=.9,minimum size=10mm] at ($(A)!.5!(E)$) {R$_A$} edge (A) edge (E);
      \node[relationship,scale=.9,minimum size=10mm] at ($(B)!.5!(E)$) {R$_B$} edge (B) edge (E);
      \node[relationship,scale=.9,minimum size=10mm] at ($(C)!.5!(E)$) {R$_C$} edge (C) edge (E);
    \end{tikzpicture}}
  \end{center}
\end{frame}