[DTB] fixing mapping in numeric functions

As defined in [1], the XQuery numeric operators are only defined if both
operands have the same.  However, as pointed out by one of our external
commentors the numeric operators in RIF assumed that these numeric
operators actually do type promotion for us. So, the numeric operators
in RIF were not well defined.
I fixed the problem by including the rules for type promotion in the
definition of the Mapping in [2].

Best, Jos

[1] http://www.w3.org/TR/2007/REC-xpath-functions-20070123/#op.numeric
[2] http://www.w3.org/2005/rules/wiki/DTB#Numeric_Functions
-- 
Jos de Bruijn            debruijn@inf.unibz.it
+390471016224         http://www.debruijn.net/