preceding and following axes do not include siblings children

[shepmaster]

With this XML:

<a0>
  <b0>
    <c0 />
    <c1 />
  </b0>
  <b1>
    <c2 />
    <c3 />
    <c4 />
  </b1>
  <b2>
    <c5 />
    <c6 />
  </b2>
</a0>

If we start at c3 and use the preceding axis, we should visit c2, c1, c0, and b0, but we currently only visit c2 and b0.

[shepmaster]

Closed via #102