Catherine: this seems to work..
```
class Unsigned(Shape):
      def __class_getitem__(cls, key: int) -> Type:
          return NewType(f"Shape_{cls.__name__}_{key}", Shape)

  class Command(data.Struct):
      class Kind(enum.Enum):
          SET_ADDR = 0
          SEND_DATA = 1

      valid: Unsigned[1]
      kind: Kind
```