Function binary 

pub fn binary<'a, L, S, F>(
    state: &mut ParserState<'a, L, S>,
    left: &'a GreenNode<'a, L>,
    op_kind: <L as Language>::TokenType,
    op_precedence: u8,
    assoc: Associativity,
    result_kind: <L as Language>::ElementType,
    parse_expr: F,
) -> &'a GreenNode<'a, L>
where
    L: Language,
    S: Source + ?Sized,
    F: FnMut(&mut ParserState<'a, L, S>, u8) -> &'a GreenNode<'a, L>,
    <L as Language>::ElementType: From<<L as Language>::TokenType>,

A helper for parsing binary (infix) expressions.