DOC: Improve the example file.

Put node types, edge types and other utilities into separate
tables. Add explicit examples of different edge types. For
instance, how to draw undirected edges (without arrow).

example.tex

@@ -63,17 +63,53 @@
       \tikz{ %
         \node[factor] [label=$\mathcal{N}$] {}; %
       }
+    \end{tabular}
+  \end{center}
+\end{table}
+\begin{table}[ht]
+  \caption{Edge types}
+  \begin{center}
+    \begin{tabular}{llc}
+      Type & Syntax & Output
       \\
-      Factor with nodes &
-      &
+      \hline
+      Directed edges &
+      \texttt{\textbackslash edge[opts]\{inputs\}\{outputs\}} &
+      \tikz{ %
+        \node[obs] (y) {$y$} ; %
+        \node[latent, left=of y, yshift=0.5cm] (mu) {$\mu$} ; %
+        \node[latent, left=of y, yshift=-0.5cm] (tau) {$\tau$} ; %
+        \edge {mu,tau} {y} ; %
+      }
+      \\
+      Undirected edges &
+      \texttt{\textbackslash edge[-,opts]\{inputs\}\{outputs\}} &
+      \tikz{ %
+        \node[obs] (y) {$y$} ; %
+        \node[latent, left=of y, yshift=0.5cm] (mu) {$\mu$} ; %
+        \node[latent, left=of y, yshift=-0.5cm] (tau) {$\tau$} ; %
+        \edge[-] {mu,tau} {y} ; %
+      }
+      \\
+      Factor graph edges &
+      \texttt{\textbackslash factoredge[opts]\{inputs\}\{via\}\{outputs\}} &
       \tikz{ %
         \node[obs] (y) {$y$} ; %
         \node[latent, left=of y, yshift=0.5cm] (mu) {$\mu$} ; %
         \node[latent, left=of y, yshift=-0.5cm] (tau) {$\tau$} ; %
         \factor[left=of y] {y-factor} {$\mathcal{N}$} {} {};
         \factoredge {mu,tau} {y-factor} {y} ; %
       }
+    \end{tabular}
+  \end{center}
+\end{table}
+\begin{table}[ht]
+  \caption{Utilities}
+  \begin{center}
+    \begin{tabular}{llc}
+      Type & Syntax & Output
       \\
+      \hline
       Plate &
       \texttt{\textbackslash plate} &
       \tikz{ %