while abort() is really only useful for listeners (in all other cases

while abort() is really only useful for listeners (in all other cases
we know the stack trace), it's better to use a separate function for
it, as this can be used conditionally, as in:

  setlistener("/the/property", func(n) {
      if (n.getValue() < 0) # who's writing negative numbers to /the/property?!
          abort();          # let's check the backtrace ...
  });