text: __all__ = ['MISSING', 'Possibly',  'Decorator', 'F']
  
  from enum import Enum
  from typing import Callable, TypeVar, Union
  
  
  # PEP-blessed solution for defining a Singleton type:
  # https://www.python.org/dev/peps/pep-0484/#id30
  class _Missing(Enum):
      flag = 'Missing'
  
  
  MISSING = _Missing.flag
  """Singleton that works as a placeholder for a missing value. Useful when None
  can't be used to play the role because it represents a valid non-null value."""
  
  _T = TypeVar('_T')
  Possibly = Union[_Missing, _T]
  """Possibly[T] is meant to be used in place of Optional[T] when None can't be
  used to represent a missing value."""
  
  F = TypeVar('F', bound=Callable)
  """Type variable for a Callable."""
  
  Decorator = Callable[[Callable], Callable]
  """Type alias for a simple function decorator."""
  