DS-3-03: ValdidityDependsUpon

I am requesting we close out this issue. Here is a brief
re-cap of its history.
 
(1) It appeared in one of the core drafts (maybe 8?). The general idea
was that it would link to "additional" conditions (critical) that would
need to be verified at the point when a RP determines assertion
validity.
 
(2) Some examples: (1) an arbitrary condition (Temperature in Alaska
less than 50 F at 2AM today), (2) reference to another assertion (S2ML
DependsUpon
element allows an attribute assertion (Q) to "point" to an AuthN assertion
(P)
with interpretation ~P => ~Q).
 
(3) The core-16 definition of <ConditionsType> allows for an zero or 
more elements of type <AbstractCondition> in an assertion. Conditions
of the type described in (2) can be modeled through extension schema
with core-16. 
 
 
-- prateek