code: def to_json_string(self, indent=None):
          ''' Convert the document to a JSON string.
  
          Args:
              indent (int or None, optional) : number of spaces to indent, or
                  None to suppress all newlines and indentation (default: None)
  
          Returns:
              str
  
          '''
          root_ids = []
          for r in self._roots:
              root_ids.append(r.id)
  
          root_references = self._all_models.values()
  
          json = {
              'title' : self.title,
              'roots' : {
                  'root_ids' : root_ids,
                  'references' : references_json(root_references)
              },
              'version' : __version__
          }
  
          return serialize_json(json, indent=indentf to_json_string(self, indent=None):
          ''' Convert the document to a JSON string.
  
          Args:
              indent (int or None, optional) : number of spaces to indent, or
                  None to suppress all newlines and indentation (default: None)
  
          Returns:
              str
  
          '''
          root_ids = []
          for r in self._roots:
              root_ids.append(r.id)
  
          root_references = self._all_models.values()
  
          json = {
              'title' : self.title,
              'roots' : {
                  'root_ids' : root_ids,
                  'references' : references_json(root_references)
              },
              'version' : __version__
          }
  
          return serialize_json(json, indent=indent)
docs: Convert the document to a JSON string.
  
          Args:
              indent (int or None, optional) : number of spaces to indent, or
                  None to suppress all newlines and indentation (default: None)
  
          Returns:
              str