Introduction
=============================================

.. graphviz::

   digraph Flatland {
   
      a -> b -> c -> g; 
      a  [shape=polygon,sides=4]
      b  [shape=polygon,sides=5]
      c  [shape=polygon,sides=6]
   
      g [peripheries=3,color=yellow];
      s [shape=invtriangle,peripheries=1,color=red,style=filled];
      w  [shape=triangle,peripheries=1,color=blue,style=filled];
      
      }