A challenge.. Group Periods of Data (1..5, 2..8, 4..9)

A challenge, group the following XML into 2 periods.  The periods are
arbitrary, but for this example they happen to be:
Period 1:  1 - 12
Period 2:  14 - 30

Expected Result:
<result>
  <period begins="1" ends="12">
    <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"/>
  </period>
  <period begins="14" ends="30">
    <B period_begin="14" period_end="16"/>
    <B period_begin="16" period_end="20"/>
    <B period_begin="16" period_end="30"/>
  </period>
</result>

Source XML / Result (sorted)
<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="16" period_end="20"/>
  <B period_begin="16" period_end="30"/>
</A>

Source XML / Result (un-sorted)
<A>
  <B period_begin="14" period_end="16"/>
  <B period_begin="2" period_end="7"/>
  <B period_begin="16" period_end="20"/>
  <B period_begin="1" period_end="5"/>
  <B period_begin="4" period_end="12"/>
  <B period_begin="16" period_end="30"/>
  <B period_begin="3" period_end="10"/>
</A>