XQuery function

I declared the function:

define function tInterval($param) returns result
{
   for $var1 in document("tvm-4.xml")//TemporalObjects/Object
   return
      <result oid={$var1/@oid}>
      {
         for $var2 in concat("$var1//Attributes/",$param,"/Value")
         return
            <tInterval tTimei={$var2/@tTimei} tTimef={$var2/@tTimef}/>
      }   
      </result>
}

And call:
tInterval(CONCAT_ELEMENT)

I want concat a child in expression, like this: "//Attributes/CONCAT_ELEMENT/Value"
A function CONCAT can used for this?
Other function can?
Or no way?

CHPGomes.