> I'm still trying to get my head around how to use the "?" operator

If you think of a map as being like an element with attributes but no name and
no children, then unary "?" behaves like unary "@", while binary "?" behaves
like "/@"

Compare

let $e := (<e x="1" y="2" z="3"/>, <e x="10" y="20" z="30"/>

let $m := (map{"x":1, "y":2, "z":3}, map{"x":10, "y":20, "z":30})

then $m?x corresponds to $s/@x (returning (1, 10))

and $m[?x = 1]?y corresponds to $e[@x = 1]/@y (returning 2)

The analogy starts to break down for dynamic references:

$m?($key) corresponds to $e/@*[name()=$key] (But you can also write it as
$m($key) to save one keystroke)

Michael Kay
Saxonica