XML design of ((a and b) or c)

I've just had to design some XML to model items that can have "and"
and "or" relationships between each one.

For example:

(a and b) or c

could be designed as:

<or>
    <a>
    <and>
        <b/>
        <c/>
    </and>
</or>

another option could be to rely on position:

<a>
<and/>
<b/>
<or/>
<c/>

and another could be model the relationships separately somehow:

<relationships>
  <rel ref="r1" type="and">
    <ent id="a"/>
    <ent id="b"/>
  </rel>
  <rel ref="r2" type="or">
    <ent id="r1"/>
    <ent id="c"/>
  </rel>
</relationships>
<a id="a"/>
<b id="b"/>
<c id="c"/>

Each has its own advantages/drawbacks.  Personally I like the first
technique, although it can get cluttered when there are 10+ items.

Are there any better ways that I'm missing?

cheers
andrew