[pet] PET code changes

[Stephan Oepen]

hi bernd,

to fix the printing of root symbols on derivations, i ended up making a
relatively invasive change: adding an optional parameter `level' to all
of the item printer methods.  could you take a look at:

  http://pet.opendfki.de/changeset/589/pet

i felt passing the level around as a parameter (rather than using state
in the various objects) was more transparent and will trivially lead to
a thread-safe solution.  if you buy into this point of view, one should
also revise the indentation mechanics now (using the new `level'), and
probably check that i did not break anything (i mostly cared about the
interactive and [incr tsdb()] derivations printing).

in principle this fix (output of root nodes) should be merged into the
trunk relatively soon.  but please see what you think of these changes
first :-).

                                                         best  -  oe

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