I always think of xml like a set data structure. Ie:

<class>
 <person>john</person>
 <person>sarah</person>
</class>

Is equivalent to:

<class>
 <person>sarah</person>
 <person>john</person>
</class>

Question One: Are these two things logicly equivalant?

Are you allowed to make things like this in xml?

<methodCall>
  <param>happy</param>
  <param>sad</param>
</methodCall>

Or do you need to do it like this:

<methodCall>
  <param arg="1">happy</param>
  <param arg="2">sad</param>
</methodCall>

Question Two: Are these two things logically equivalent?

Question Three: Is xml usually treated like a set or a list?