Function expression_impl 

fn expression_impl<I, O, Pop, Pre, Post, Pix, E>(
    i: &mut I,
    parse_operand: &mut Pop,
    prefix: &mut Pre,
    postfix: &mut Post,
    infix: &mut Pix,
    min_power: i64,
) -> Result<O, E>
where
    I: Stream + StreamIsPartial,
    Pop: Parser<I, O, E>,
    Pix: Parser<I, Infix<I, O, E>, E>,
    Pre: Parser<I, Prefix<I, O, E>, E>,
    Post: Parser<I, Postfix<I, O, E>, E>,
    E: ParserError<I>,

Opaque implementation of the Pratt parser.