Tabling: cleanup of backtrace

To help out with the cleaning up of the backtrace:

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1 ?- a() =.. [F|A].
ERROR: Domain error: `compound_non_zero_arity' expected, found `a()'
ERROR:    [9] a()=..[_6754|_6756]
ERROR:    [8] '$tabling':'$wfs_call'(user:(... =.. ...),user:_6806) at /usr/lib/swipl/boot
2 ?-

I think the $wfs_call should not show up.

This is fixed in the current source.

Great, tnx!