[Core] updated safeness condition

I updated the safeness condition to fix the technical problem I noticed
during the telephone conference and to improve the presentation.  I hope
it is now easier to follow.
Please let me know if there are still problems in the presentation.

The problem with the previous version of the definition was that it
allowed to assign "bound" to all variables, even those appearing only in
external terms.
I now make a distinction between safe and strongly safe variables, where
the strongly safe variables are those that are made safe by non-external
atoms.  It is required that all variables that are not strongly safe are
assigned "unbound".

Please *read carefully* and criticize.
And I remind you that it is not necessary to wait until the next
telephone conference before starting to read the definition.


Best, Jos

[1] http://www.w3.org/2005/rules/wiki/Core#Safeness
-- 
Jos de Bruijn            debruijn@inf.unibz.it
+390471016224         http://www.debruijn.net/
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