FYI: glr: Trace svals and lvals in reductions

[Akim Demaille]

I will add this to the other skeletons too.  There are a couple
of minor issues.

- I would like to make yydebug more than Boolean to control what
  people want to see,

- the traces obviously are not able to take requests such as $<val>2
  into account.  I don't plan to work on this.

- I have removed a couple of m4_eval that used to transform 3 - 1 into
  2 because I need to be able to address non-static values.  I doubt
  there are compilers that don't perform this simple kind of  
optimization.

diffs.patch
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