def is_atomic(x) -> bool:
    """
    Return ``True`` if *x* is a valid atomic value.

    Examples:
        >>> from penman.tree import is_atomic
        >>> is_atomic('a')
        True
        >>> is_atomic(None)
        True
        >>> is_atomic(3.14)
        True
        >>> is_atomic(('a', [('/', 'alpha')]))
        False
    """
    return x is None or isinstance(x, (str, int, float))