You can merge two sequences into a single data sequence. This pattern is common when the data sequences are in an array (that is, the sequence of data items of compatible types).The two append operations shown below under assign demonstrate how to merge data sequences:

<assign>
    <!-- initialize "mergedLineItems" variable
         to an empty element -->
    <copy>
        <from> <p:lineItems /> </from>
        <to variable="mergedLineItems" />
    </copy>
    <bpelx:append>
          <bpelx:from variable="input" part="payload"
                query="/p:invoice/p:lineItems/p:lineitem" />
          <bpelx:to variable="mergedLineItems" />
    </bpelx:append>
    <bpelx:append>
          <bpelx:from variable="literalLineItems"
                query="/p:lineItems/p:lineitem" />
          <bpelx:to variable="mergedLineItems" />
    </bpelx:append>
</assign>