def __init__(self, content: Optional[BasePattern] = None) -> None:
        """
        Initializer.

        The argument is either a pattern or None.  If it is None, this
        only matches an empty sequence (effectively '$' in regex
        lingo).  If it is not None, this matches whenever the argument
        pattern doesn't have any matches.
        """
        if content is not None:
            assert isinstance(content, BasePattern), repr(content)
        self.content = content

        Initializer.

        The argument is either a pattern or None.  If it is None, this
        only matches an empty sequence (effectively '$' in regex
        lingo).  If it is not None, this matches whenever the argument
        pattern doesn't have any matches.
        
src/blib2to3/pytree.py