docs: def validate_none(b):
      """Validate that None is given
  
      Parameters
      ----------
      b: {None, 'none'}
          None or string (the case is ignored)
  
      Returns
      -------
      None
  
      Raises
      ------
      ValueError"""
      if isinstance(b, six.string_types):
          b = b.lower()
      if b is None or b == 'none':
          return None
      else:
          raise ValueError('Could not convert "%s" to None' % b)