9. A method of managing access to a resource, the method comprising: abducing, from first information that comprises (a) a query and (b) a policy implemented by a guard that controls access to the resource, a first answer set that comprises a first assertion set, said first answer set comprising a first assertion, a first variable, and a first constraint that have been chosen, said first assertion set comprising said first assertion; receiving, by said guard, a second assertion set that comprises either (a) said first assertion set, or (b) a third assertion set that is derived from said first assertion set by substituting at least one variable in place of at least one principal named in said first assertion set; determining, by said guard, that said query is true under said policy in the presence of second information that comprises said second assertion set; and based on said determining, allowing an entity to access the resource.
14. The method of claim 9 , further comprising: using a filter to determine whether said query has abducible sub-goals and to govern what types of assertions may be included in said first answer set.