\frametitle{Relationship Sets}

    A \emph{relationship \alert{set}} is set of relationships of the same kind.
  That is, a \emph{relationship set} is a set of tuples $(e_1,e_2,\ldots,e_n)$
  where $e_1 \in E_1$, \ldots, $e_n \in E_n$ are from entity sets $E_1,\ldots,E_n$.
    Example of a relationship set
      \{ \; &(\text{Uma Thurman},\,\text{Pulp Fiction}),\;
            (\text{Mark Hamill},\,\text{Star Wars}),\\
            &(\text{Harrison Ford},\,\text{Star Wars}), \;
            (\text{Harrison Ford},\,\text{Indiana Jones}) \;\}
    The elements of a relationship set are relationships:
      \item $(\text{Mark Hamill},\,\text{Star Wars}) \in \text{shows}$\; is a relationship
  A relationship set \textit{plays-in} between 
  entity sets \textit{actor} and \textit{movie} is indicated as follows in E/R models:
    \begin{tikzpicture}[every edge/.style={link}]
      \node[entity] (actor) {actor};
      \node[entity,right of=actor, node distance=6cm] (movie) {movie};

      \node[relationship] (plays) at ($(actor)!.5!(movie)$) {plays-in} edge (actor) edge (movie);