docs: def opt_key(spec):
      """
      Returns a validator which allows the value to be missing.
      Similar to :func:`optional` but wraps a string in
      :class:`~monk.validators.Equals` instead of :class:`~monk.validators.IsA`.
      Intended for dictionary keys.
  
      ::
  
          >>> opt_key(str) == IsA(str) | ~Exists()
          True
          >>> opt_key('foo') == Equals('foo') | ~Exists()
          True
  
      """
      if isinstance(spec, text_types):
          spec = Equals(spec)
      return optional(spec)