You are right, of course. Thanks for pointing it out.
I absentmindedly copied the sequence from the result of an early script where the nodes were in that order. Using the same structure that we have in the company example, the post-order traversal is correct. Just change "callback" into "callbackback" and the printout will be:

    net empl:  6  budget:    25000
    str empl:  8  budget:    65000
  sales empl: 14  budget:    90000
  pers  empl:  4  budget:    10000
    res empl: 10  budget:   100000
    dev empl: 15  budget:    90000
  R&D   empl: 25  budget:   190000
company empl: 43  budget:   290000
              |
          <company>
     /--------+---------\
     |        |         |
  <sales>   <pers>    <R&D>
  /-----\            /-----\
  |     |            |     |
<net> <str>        <res> <dev>
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