OSM7 how to write the xq in Order State Policy

Question

In Order State Policy I set for
     state:In Progress Order State
     request: Cancel Order     
a xq code in the Condition Transiction form, which check a xml field and return true or false base on the content, but it does not works.
Now, can I log the input of Order State Policy and how can I debug the xq?
Please helpme to undestund how to write and debug the xq code?
below my xq:
declare namespace saxon="http://saxon.sf.net/";
declare namespace xsl="http://www.w3.org/1999/XSL/Transform";
let $taskData := fn:root(.)/GetOrder.Response
let $ponr := $taskData/_root/PONR
let $ponr_value := fn:normalize-space( $ponr/text() )
return
if (fn:exists($taskData) )
then (
if ("NO" = $ponr_value) then (
true()
else (
false()
else(
     false()
input example:
<GetOrder.Response>
<_root
<PONR>NO</PONR>

Answer 1

So, I can debug the xq adding the line below step by step...
fn:trace($taskData, "label")
Below my final xq that I put on the policy.
declare namespace saxon="http://saxon.sf.net/";
declare namespace xsl="http://www.w3.org/1999/XSL/Transform";
let $taskData := fn:root(.)/GetOrder.Response
let $ponr := $taskData/_root/PONR
let $ponr_value := fn:normalize-space( $ponr/text() )
return
if (fn:exists($taskData) )
then (
if ( $ponr_value = "NO" ) then (
true()
else (
          fn:error(fn:QName("urn:com:metasolv:oms:xmlapi:1", "PointOfNoReturn"), "PONR reached.")
else(
     fn:error(fn:QName("urn:com:metasolv:oms:xmlapi:1", "PointOfNoReturn"), "PONR reached taskData does not exist.")
)