fn parse_negation<'i, 't, P, Impl>(
    parser: &P,
    input: &mut Parser<'i, 't>,
    state: SelectorParsingState,
) -> Result<Component<Impl>, ParseError<'i, P::Error>>where
    P: Parser<'i, Impl = Impl>,
    Impl: SelectorImpl,Expand description
Level 3: Parse one simple_selector.  (Though we might insert a second
implied “