code: def set_edge_label(self, edge, label):
          
          self.set_edge_properties(edge, label=label )
          if not self.DIRECTED:
              self.set_edge_properties((edge[1], edge[0]) , label=label )
docs: Set the label of an edge.
  
          @type  edge: edge
          @param edge: One edge.
  
          @type  label: string
          @param label: Edge label.