code: def _missing_(cls, value):
          
          if not (isinstance(value, int) and 0x00 <= value <= 0xFF):
              raise ValueError('%r is not a valid %s' % (value, cls.__name__))
          if 0x0D <= value <= 0xEF:
              extend_enum(cls, 'Unassigned [0x%s]' % hex(value)[2:].upper().zfill(2), value)
              return cls(value)
          if 0xF0 <= value <= 0xFF:
              # [RFC 7540]
              extend_enum(cls, 'Reserved for Experimental Use [0x%s]' % hex(value)[2:].upper().zfill(2), value)
              return cls(value)
          super()._missing_(value)
docs: Lookup function used when value is not found.