I’m struggling conceptually with using hxt.

I have an input xml document with a fragment like this:

      <Slip>
        <BNFY_NM>
          <snm>SURNAME</snm>
          <gvn_nm>GIVENAME</gvn_nm>
        </BNFY_NM>
        <SEC_BNFY_NM>
          <sec_snm>SMITH</sec_snm>
          <sec_gvn_nm>JANE</sec_gvn_nm>
        </SEC_BNFY_NM>
        ...

I need to insert another element as the first child of <Slip>, like this:

      <Slip>
        <SlipSupplementary>
          <acct_nbr>SURNAME GIVENAME</acct_nbr>
        </SlipSupplementary>
        <BNFY_NM>
          <snm>SURNAME</snm>
          <gvn_nm>GIVENAME</gvn_nm>
        </BNFY_NM>
        ...

The new element has content based on the existing child elements of the parent node. I can get it working if I just need to insert a static node, but I am struggling to get the content of the other nodes into the new one.

testHXT ::  IO ()
testHXT = do
  runX $ readDocument [withValidate no] "input.xml"
         >>>
         processChildren (processSlips `when` isElem)
         >>>
         writeDocument [withIndent yes] "output.xml"
  return ()


processSlips :: ArrowXml a => a XmlTree XmlTree
processSlips = processTopDown (addAccountNo `when` isSlip)
    where isSlip = isElem >>> hasName "Slip"
          addAccountNo = replaceChildren (acctnoElement <+> getChildren)
          acctnoElement = mkelem "SlipSupplementary" [] [mkelem "dlr_acct_nbr" [] [txt $ L.concat [snm," ", gnm]]]
          snm = deep (isElem >>> hasName "snm" >>> getChildren >>> getText)
          gnm = deep (isElem >>> hasName "gnm" >>> getChildren >>> getText)

The error code is this:

   * Couldn't match type `[Char]' with `Char'
      Expected: [Char]
        Actual: a0 (Data.Tree.NTree.TypeDefs.NTree XNode) String
    * In the expression: snm
      In the first argument of `concat', namely `[snm, gnm]'
      In the second argument of `($)', namely `concat [snm, gnm]'
    |
    |           acctnoElement = mkelem "SlipSupplementary" [] [mkelem "dlr_acct_nbr" [] [txt $ L.concat [snm,gnm]]]

My sense is that I need to make snm and gnm use input XmlTree’s from the parent node (like what getChildren is producing), but how would I do that?

[I’ve tried variations like below, but have also run into a brick wall, and regardless, I’m not sure this will scale well if I need to use content from six sibling elements:]

        acctnoElement = 
              isElem 
              >>>
              deep (isElem >>> hasName "snm" >>> getChildren >>> getText)
              >>>
              arr (snm -> mkelem "SlipSupplementary" [] [mkelem "dlr_acct_nbr" [] [txt snm]])