Concatenate values in ST Transformation?

I am writing a serialization through ST and it should put several vars/nodes into one XML tag.

Input ABAP itab that should be serialized:

ROW_ID       ROW
               VAL       INDEX
  1            val55        X
               val32        Y
               val46        X

  2            val8         X
               val16        
               val789        

  3            val78        Y
               val53        Y
               val98        Y

where each ROW value is an itab which consists of VAL and INDEX fields

Expected output sample for first row:

  <rows>
    <row r="1">
        <c r="1_xcell1">
            <v>val55</v>
        </c>
        <c r="1_ycell2">
            <v>val32</v>
        </c>
        <c r="1_xcell3">
            <v>val46</v>
        </c>
     <row>
   </rows>

Here

1 in <row r="1"> corresponds to row number from ROW_ID

1_xcell1 in <c r="1_xcell1"> is a concatenation of ROW_ID from the current line of root table, INDEX field of current ROW line, literal cell and loop counter of ROW table

The transformation I ended up with is:

<?sap.transform simple?>
<tt:transform xmlns:tt="http://www.sap.com/transformation-templates" template="main">
  <tt:root name="root"/>
  <tt:variable name="range" val="11"/>
  <tt:template name="main">
      <rows>
        <tt:loop name="row" ref="root">
          <row>
            <tt:attribute name="r" value-ref="row_id"/>
            <tt:assign to-var="range" ref="row_id"/>
            <tt:loop name="cells" ref="$row.ROW">
            <tt:serialize>
              <c>
                <tt:attribute name="r"><tt:write var="range" map="val(I(1)) = xml('1_xcell1'), val(I(2)) = xml('2_xcell2'), val(I(3)) = xml('3_xcell3)"/></tt:attribute>
                <v><tt:value ref="value"/></v>
              </c>
            </tt:serialize>
            </tt:loop>
          </row>
        </tt:loop>
      </rows>
  </tt:template>
</tt:transform>

where I put the ROW_ID into var for using in mapping on the lower lever in <c> tag.

As you can see, this approach will not work because I have no idea how to put loop counter and how to concatenate it with other values.

I found a couple of old threads on answers.sap.com (1, 2, 3), but they are left unanswered.

Is there a concatenation in ST like in XSLT:

`<xsl:element name="{concat($segment2, '_', $tail2)}">`?

Is there a built-in variable for loops like SY-TABIX in ABAP?

This should work (if $row.row_id equals 1, it will generate ...<c r="1_xcell1">...):

            ...
              <c>
                <tt:attribute name="r">
                  <tt:value ref="$row.row_id"/>_xcell<tt:value ref="$row.row_id"/>
                </tt:attribute>
              ...

Concerning the loop counter, I think it can't be done within Simple Transformations, your solution to pass the row number explicitly inside the internal table is the best.