comment créer des expression

bonjour
j'aimerai créer une expression xquery
mais je sais pas la syntaxe
par exemple j'ai trouvé:

VWeightDistanceExpr ::= Expr “vweightdistance”
VDistanceWithWeight
where VDistanceWithWeight specifies how the final similarity is calculated with
added notion of weights. The syntax of the VDistanceWithWeight are as follows.

VDistanceWithWeight ::= VDistanceExpr “weight” Expr
|VDistanceWithWeight ‘&&’
VDistanceWithWeight “norm” Expr

The VWeightDistanceExpr expression also returns a sequence of xs:double, which
indicates the final similarity calculated with added notion of weights in the composite
query. An example of the composite query using the VWeightDistanceExpr
expression is given below. It uses a weight of 0.6 for the average histogram and a
weight of 0.4 for the edge histogram feature. Moreover, the distance of each feature is
normalized by the factor α .

let $h := (20,14,…,26)
$m := (27,12,…,25)
for $node in //Segment
let $distance := vweightdistance
$node/GoFGoPHistogramD/Histogram/HistogramValues
eu $h weight 0.4
&& $node/EdgeHistogramD/Histogram/HistogramValues
eu $m weight 0.6 norm α
where $distance < T
return <Segment>
<SegmentId>{$[node]/@id}<SegmentId>
<eu>{$distance}</eu>
</Segment>

j'ai pas compris la différence entre l'utilité de la première expression , puisqu'il as intégrer dans une fonction, et j'ai pas compris le syntax qu'il las suit pour construire cette expression.

merci beaucoup