[basex-talk] Strange "group by" behavior

[Alexander Krasnogolowy]

Given the following XQuery:
let $input := <root>
                <item><key>a</key><value>1</value></item>
                <item><key>b</key><value>2</value></item>
                <item><key>b</key><value>3</value></item>
              </root>

let $map :=  map:merge((
  map:entry(1, "A"),
  map:entry(2, "B"),
  map:entry(3, "C")
))

for $result in $input/*
let $key := $result/key/text()
let $value := $result/value/text()
group by $key
return map:get($map, $key)

When trying to execute that query, I get the error "Item expected, sequence 
found: (map {1:"A",2:"B",3:"C"}, map {1:"A",2:"B",3:"C"}).
But why does it group the map? It isn't part of that for-loop but rather some 
kind of global variable. Or do I misunderstand the "group by" clause?
I only expect the $value to be grouped which results in a sequence.

Thanks,
Alex