generate powerset

powerset.xq

(: let $combs:=[(),1,2,3,(1,2),(2,3),(1,3),(1,2,3)] :)

declare function local:powerset($items as item()* )
as array(*){
if (count($items)=0)  then [()]
else let $x:=local:powerset(tail($items))
     return array:join(($x, 
                        array:for-each($x,function($v){head($items),$v}))
                      )
};
local:powerset(1 to 3) =>array:for-each(count#1