Function postfix 

pub fn postfix<'a, L, S>(
    state: &mut ParserState<'a, L, S>,
    left: &'a GreenNode<'a, L>,
    op_kind: L::TokenType,
    result_kind: L::ElementType,
) -> &'a GreenNode<'a, L>
where
    L: Language,
    S: Source + ?Sized,
    L::ElementType: From<L::TokenType>,

A helper for parsing postfix expressions.