code: def to_json(self):
          ''' Returns the JSON representation of this graph. '''
          roots = []
          for r in self.roots:
              roots.append(r.to_json())
          return {'roots': rootsf to_json(self):
          ''' Returns the JSON representation of this graph. '''
          roots = []
          for r in self.roots:
              roots.append(r.to_json())
          return {'roots': roots}
docs: Returns the JSON representation of this graph.