I would like to select the purchase-node with the attribute pgnr, which has the value from another pgnr attribute, startig with "KEY", concatinated with "c".

Example:

```
<root>
<purchact hhid="xxx">
  <purchase pgnr="41">
   <purchvalues field_name="Number" field_value="1"/>
  </purchase>
  <purchase pgnr="KEY9802">
   <purchvalues field_name="Number" field_value="2"/>
  </purchase>
 <purchase pgnr="9802c">
   <purchvalues field_name="Number" field_value="3"/>
  </purchase>
 </purchact>
</root>

```

In this case, I am looking for the purchase-node with the pgnr-attribute "9802c", because the purchase-node with the pgnr-attribute starting with "KEY" has as the following characters "9802".

I tried 

```
root/purchact/purchase[@pgnr=concat(substring-after(@pgnr, "KEY"), "c")]

```

but it doesn't work. 

Could anybody help? Thanks so much!