Function map_key_eq 

pub fn map_key_eq<I: DocIndexLike>(a: &Value, b: &Value, idx: &I) -> bool

XPath 3.1 §17.1 map-key equality: numeric keys compare numerically (1 = 1.0), with NaN equal to itself; otherwise compare by string value (covering string / untyped / anyURI / boolean keys).