Function begingroup 

pub fn begingroup<'a, Pos: StringPosition, T: FromLaTeXToken<'a, Pos>, State: ParserState<'a, Pos, T>>(
    begingroup: Macro<'a, Pos>,
    p: &mut LaTeXParser<'a, Pos, T, State>,
) -> MacroResult<'a, Pos, T>