docs: def value_attr(attr_name):
      """
      Creates a getter that will retrieve value's attribute
      with specified name.
      @param attr_name: the name of an attribute belonging to the value.
      @type attr_name: str
      """
  
      def value_attr(value, context, **_params):
          value = getattr(value, attr_name)
          return _attr(value)
  
      return value_attr