Node lifetime is shorter than expected due to let variable inlining

A query like
let $x:=<a/>
return by-ref(ref($x))

gets rewritten (in some case, depending on the complexity of the expression and number of references to the variable) as:
by-ref(ref(<a/>))

Thus the node has been destroyed when by-ref is called.

The following test named reference_5.xq:
import module namespace idd = "http://www.zorba-xquery.com/reference" at "reference.xqlib";
import module namespace id = "http://www.zorba-xquery.com/modules/node-reference";
import module namespace doc = "http://www.zorba-xquery.com/modules/store/dynamic/documents";

variable $node:=<root>global</root>;

fn:put(<root>doc</root>,"doc");

<result>
<temporary-variable-in-scope>{let $temp:=<root>temp</root> return id:node-by-reference(id:node-reference($temp))}</temporary-variable-in-scope>
<temporary-variable-in-scope>{
let $temp:=<root>temp</root>
return
let $ref:=id:node-reference($temp)
return
id:node-by-reference($ref)
}</temporary-variable-in-scope>
<temporary-variable-in-scope>{
for $i in (1 to 3)
let $temp:=<root>temp</root>
let $ref:=id:node-reference($temp)
return
id:node-by-reference($ref)
}</temporary-variable-in-scope>

<temporary-variable-in-scope>{let $temp:=<root>temp</root> return id:node-by-reference(id:node-reference($temp))}</temporary-variable-in-scope>

</result>

Should return:
<?xml version="1.0" encoding="UTF-8"?>
<result>
  <temporary-variable-in-scope>
    <root>temp</root>
  </temporary-variable-in-scope>
  <temporary-variable-in-scope>
    <root>temp</root>
  </temporary-variable-in-scope>
  <temporary-variable-in-scope>
    <root>temp</root>
    <root>temp</root>
    <root>temp</root>
  </temporary-variable-in-scope>
  <temporary-variable-in-scope>
    <root>temp</root>
  </temporary-variable-in-scope>
</result>

Instead it returns:
<?xml version="1.0" encoding="UTF-8"?>
<result>
  <temporary-variable-in-scope/>
  <temporary-variable-in-scope/>
  <temporary-variable-in-scope/>
  <temporary-variable-in-scope/>
</result>