code: def supports_undefined(self):
          
          # Test
          try:
              yes = self(const.UNDEFINED) is not const.UNDEFINED
          except (Invalid, SchemaError):
              yes = False
  
          # Remember (lame @cached_property)
          self.__dict__['supports_undefined'] = yes
          return yes
docs: Test whether this schema supports Undefined.
  
          A Schema that supports `Undefined`, when given `Undefined`, should return some value (other than `Undefined`)
          without raising errors.
  
          This is designed to support a very special case like that:
  
          ```python
          Schema(Default(0)).supports_undefined  #-> True
          ```
  
          This way a validator can declare that it has a default in case no value was provided,
          and this case happens when:
  
          1. A [`Required`](#required) mapping key was not provided, and it's mapped to `Default()`
          2. .. no more supported cases. Yet.
  
          :rtype: bool