docs: def make_selector(value):
      '''Create a selector callable from the supplied value.
  
      Args:
          value: If is a callable, then returned unchanged.  If a string is used
              then create an attribute selector. If in an integer is used then
              create a key selector.
  
      Returns:
          A callable selector based on the supplied value.
  
      Raises:
          ValueError: If a selector cannot be created from the value.
      '''
      if is_callable(value):
          return value
      if is_string(value):
          return a_(value)
      raise ValueError("Unable to create callable selector from '{0}'".format(value))