code: def add_meta(self, name, value):
          
          for mt in self.metas:
              if mt.name == name:
                  mt.value = value
                  return self
          self.metas.append(MetaDef(name, value))
          return self
docs: Add a pair of meta data to the definition
  
          :param name: name of the meta
          :type name: str
          :param value: value of the meta
          :type value: str