code: def to_gremlin(self):
          
          self.validate()
          if self.optional:
              operation = u'optional'
          else:
              operation = u'back'
  
          mark_name, _ = self.location.get_location_name()
  
          return u'{operation}({mark_name})'.format(
              operation=operation,
              mark_name=safe_quoted_string(mark_name))
docs: Return a unicode object with the Gremlin representation of this BasicBlock.