code: def str2bool_or_none(string_, default='raise'):
      
      if is_none(string_, default=False):
          return None
      else:
          return str2bool(string_, default)
docs: Convert a string to a bool or to None.
  
      Parameters
      ----------
      string_ : str
      default : {'raise', False}
          Default behaviour if none of the "true" or "none" strings is detected.
  
      Returns
      -------
      bool_or_none : bool or None
  
      Examples
      --------
      >>> str2bool_or_none('True')
      True
      >>> str2bool_or_none('1')
      True
      >>> str2bool_or_none('0')
      False
      >>> str2bool_or_none('undefined')