David, I found a bug.

For two contiguous elements who's period_end date match and are the
end of a period you will get the output of the current period being
processed and the additional who's period_end date is a duplicate.

(period_end='16' is an example of this)

Given:

<A>
  <B period_begin="1" period_end="5"/>
  <B period_begin="2" period_end="7"/>
  <B period_begin="3" period_end="10"/>
  <B period_begin="4" period_end="12"/>
  <B period_begin="14" period_end="16"/>
  <B period_begin="15" period_end="16"/>
  <B period_begin="52" period_end="62"/>
</A>

You get:

<result>
  <period begins="1" ends="12" />
  <period begins="14" ends="16" />
  <period begins="15" ends="16" />
  <period begins="52" ends="62" />
  <period begins="63" ends="150" />
</result>

Expected Result is:
<result>
  <period begins="1" ends="12" />
  <period begins="14" ends="16" />
  <period begins="52" ends="62" />
  <period begins="63" ends="150" />
</result>

Any ideas?