Returns the value associated with a supplied key in a given map.
map:get( $map as map(*),$key as xs:anyAtomicTypeitem()*The function map:get attempts to find an entry within the map supplied as $map that has the same key as the supplied
value of $key. If there is such an entry, it returns the associated value;
otherwise it returns an empty sequence.
let $week := map{0:"Sonntag", 1:"Montag", 2:"Dienstag",
3:"Mittwoch", 4:"Donnerstag", 5:"Freitag", 6:"Samstag"}
The expression map:get($week, 4) returns "Donnerstag".
The expression map:get($week, 9) returns ().
The expression map:get(map:entry(7,()), 7) returns ().
A return value of () from map:get could indicate that
the key is present in the map with an associated value of (), or it could
indicate that the key is not present in the map. The two cases can be distinguished by
calling map:contains.
Invoking the map as a function item has the same effect
as calling get: that is, when $map is a map, the expression
$map($K) is equivalent to map:get($map, $K). Similarly, the
expression map:get(map:get(map:get($map, 'employee'), 'name'), 'first') can
be written as $map('employee')('name')('first').
