docs: def to_dot(self):
          """
          Produce a graph in DOT format.
  
          """
          edge_labels = {
              edge.id: edge.annotation
              for edge in self._edges
          }
  
          edges = [self._format_edge(edge_labels, edge) for edge in self._edges]
  
          vertices = [
              DOT_VERTEX_TEMPLATE.format(
                  vertex=vertex.id,
                  label=dot_quote(vertex.annotation),
              )
              for vertex in self.vertices
          ]
  
          return DOT_DIGRAPH_TEMPLATE.format(
              edges="".join(edges),
              vertices="".join(vertices),
          )