Can I retrieve the element in sequence?

[On <ko...@cecid.hku.hk>]

Assume there are 3 element under the root element
<root>
    <a>abc</a>
    <b>123</b>
    <a>def</a>
</root>

Is there any method to let me know the position of element b is the second one.
or say, get all the children in sequence so that i can get the value one by one.
thanks