Nest two Xqueries for aggregation of the output over xml documents collection

Question

i have multiple xml docs, each of them has multiple LightSpot and EquipmentType. I would like to extract a single aggregated value over all the documents. So i tried to nest two queries in order to apply a double aggregation. The second query aggregation returns empty values for NumberOfLightSpots and EquipmentType even though if i return the variable $grouped_sheet the output not aggregated is displayed.

This is the code i used in eXist-db

let $grouped_sheets := for $sheet in collection('/db/light')
                 return 
                    <sheet
                        NumberOfLightSpots ="{count($sheet//LightSpot)}"
                        EquipmentType ="{distinct-values($sheet//EquipmentType)}"
                    />
for $grouped_sheet in $grouped_sheets
return  <sheet 
           NumberOfLightSpots ="{count($grouped_sheet//NumberOfLightSpots)}"
           EquipmentType ="{distinct-values($grouped_sheet//EquipmentType)}"
        />

The final return value is empty, in fact i have this output:

<sheet NumberOfLightSpots="0" EquipmentType=""/>
<sheet NumberOfLightSpots="0" EquipmentType=""/>
<sheet NumberOfLightSpots="0" EquipmentType=""/>
....

Thanks

Edit:

The structure of the return should be:

 <sheet>
   <NumberOfLightSpots>{count($sheet//LightSpot)</NumberOfLightSpots>
   <EquipmentType>{distinct-values($sheet//EquipmentType)}</EquipmentType>
 </sheet>