Changes from Algernon v1.3.3 to v2.0

1. The rule activation and execution model has been formulated more precisely.
   • Rules may not fire in exactly the same order as in Algernon 1.3.3.
   • Continuations are created for every antecedent clause, but no duplicate continuations are created.
2. The user can exercise more control over reasoning, via the following variables:
   • control whether continuations are created for different types of rules:
     • *IF-ADDED-CONTINUATIONS* (default: T)
     • *IF-NEEDED-CONTINUATIONS* (default: T)
   • enable or disable if-added (forward-chaining) and if-needed rules (backward-chaining) rules.
     • *FORWARD-CHAIN* (default: T)
     • *BACKWARD-CHAIN* (default: T)
   • Perform reasoning breadth-first or depth-first
     • *PATH-SEARCH-TYPE* (default :BREADTH)
   • Perform exhaustive or limited inference
     • *EXHAUSTIVE-INFERENCE* (default: T)
3. A new metering package has been added that counts the number of facts stored, rules fired, etc.
   • (meter-on) and (meter-off) enable/disable metering.
   • (meter-reset) resets all counters to zero.
   • (meter-stats) prints out statistics.
4. Several obsolete commands are no longer supported:

       :LISP                     use :EVAL instead.
       :CREATE                   use :A instead.
       :IN-OWN-PARTITION         obsolete.
       :DEL-RULE,  :DEL-RULES    obsolete.
       :DEL-SRULE, :DEL-SRULES   obsolete.
   

5. :BACKLINK and :INVERSE are now obsolete slot modifiers.
   • Replace :BACKLINK slot modifiers with an if-added rule:

       OLD:       (:slot imp-superset (sets sets)  :backlink subset)
     
       NEW:       (:slot imp-superset (sets sets))
                  (:srule (:slot imp-superset)
                          ((imp-superset ?s1 ?s2)
                           ->
                           (subset ?s2 ?s1)))
     

   • Replace :INVERSE modifiers with an INVERSE relation.

       OLD:       (:slot superset (sets sets)  :inverse subset)
     
       NEW:       (:slot superset (sets sets))
                  (inverse superset subset)   ;;; <--- add this
     

6. Fixed bug in the :NO-COMPLETION special form. It now accepts the form (:NO-COMPLETION . path) as previously documented.

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