Added IPC 1998 Gripper example instances.

instances/ipc-1998-gripper/domain.pddl

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+(define (domain gripper-strips)
+   (:predicates (room ?r)
+		(ball ?b)
+		(gripper ?g)
+		(at-robby ?r)
+		(at ?b ?r)
+		(free ?g)
+		(carry ?o ?g))
+
+   (:action move
+       :parameters  (?from ?to)
+       :precondition (and  (room ?from) (room ?to) (at-robby ?from))
+       :effect (and  (at-robby ?to)
+		     (not (at-robby ?from))))
+
+
+
+   (:action pick
+       :parameters (?obj ?room ?gripper)
+       :precondition  (and  (ball ?obj) (room ?room) (gripper ?gripper)
+			    (at ?obj ?room) (at-robby ?room) (free ?gripper))
+       :effect (and (carry ?obj ?gripper)
+		    (not (at ?obj ?room)) 
+		    (not (free ?gripper))))
+
+
+   (:action drop
+       :parameters  (?obj  ?room ?gripper)
+       :precondition  (and  (ball ?obj) (room ?room) (gripper ?gripper)
+			    (carry ?obj ?gripper) (at-robby ?room))
+       :effect (and (at ?obj ?room)
+		    (free ?gripper)
+		    (not (carry ?obj ?gripper)))))
+

instances/ipc-1998-gripper/prob01.pddl

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+(define (problem strips-gripper-x-1)
+   (:domain gripper-strips)
+   (:objects rooma roomb ball4 ball3 ball2 ball1 left right)
+   (:init (room rooma)
+          (room roomb)
+          (ball ball4)
+          (ball ball3)
+          (ball ball2)
+          (ball ball1)
+          (at-robby rooma)
+          (free left)
+          (free right)
+          (at ball4 rooma)
+          (at ball3 rooma)
+          (at ball2 rooma)
+          (at ball1 rooma)
+          (gripper left)
+          (gripper right))
+   (:goal (and (at ball4 roomb)
+               (at ball3 roomb)
+               (at ball2 roomb)
+               (at ball1 roomb))))

instances/ipc-1998-gripper/prob02.pddl

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+(define (problem strips-gripper-x-2)
+   (:domain gripper-strips)
+   (:objects rooma roomb ball6 ball5 ball4 ball3 ball2 ball1 left right)
+   (:init (room rooma)
+          (room roomb)
+          (ball ball6)
+          (ball ball5)
+          (ball ball4)
+          (ball ball3)
+          (ball ball2)
+          (ball ball1)
+          (at-robby rooma)
+          (free left)
+          (free right)
+          (at ball6 rooma)
+          (at ball5 rooma)
+          (at ball4 rooma)
+          (at ball3 rooma)
+          (at ball2 rooma)
+          (at ball1 rooma)
+          (gripper left)
+          (gripper right))
+   (:goal (and (at ball6 roomb)
+               (at ball5 roomb)
+               (at ball4 roomb)
+               (at ball3 roomb)
+               (at ball2 roomb)
+               (at ball1 roomb))))

instances/ipc-1998-gripper/solutions

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+instance | minimal horizon | #solutions with minimal horizon
+============================================================
+prob01.pddl | 11 | 384
+prob02.pddl | 17 | 46080