Function metacommand 

fn metacommand<'a, 'b, I>(
    grammar: &Grammar<'a, 'b, I>,
) -> impl Parser<'a, I, ManuscriptMetaCommand, Full<Rich<'a, Token>, State<'a>, ()>> + use<'a, 'b, I>
where
    I: Input<'a, Token = Token, Span = SimpleSpan> + ValueInput<'a>,