[Core] new safeness condition

I completed ACTION-687: Write a proposed new definition of the safeness
restriction

http://www.w3.org/2005/rules/wiki/Core#Safeness

Please criticize.

I suspect it can be a little more concisely.  When I find some time I
will go over it again.


I also invite anyone who is interested to go over the functions and
predicates in DTB and check whether the binding patterns defined are
appropriate.

Best, Jos
-- 
Jos de Bruijn            debruijn@inf.unibz.it
+390471016224         http://www.debruijn.net/
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