Now that I know how to start with an empty associative array and fill it: what about the other way around and remove an entry form an associative array?
i.e. how to remove the element with key b in the code below:
:global rot13 ({});
:set ($rot13->"a") "n"
:set ($rot13->"b") "o"
:set ($rot13->"c") "p"
–jeroen