code: def to_gremlin(self):
          
          self.validate()
  
          mark_name, field_name = self.location.get_location_name()
  
          if field_name is not None:
              validate_safe_string(field_name)
              if '@' in field_name:
                  template = u'm.{mark_name}[\'{field_name}\']'
              else:
                  template = u'm.{mark_name}.{field_name}'
          else:
              template = u'm.{mark_name}'
  
          validate_safe_string(mark_name)
  
          return template.format(mark_name=mark_name, field_name=field_name)
docs: Return a unicode object with the Gremlin representation of this expression.