(def world2
  '#{(connects G1 G2)
     (connects G2 G1)
     (connects G2 G3)
     (connects G3 G2)
     (on ball table1)
     (on nil table2)
     (on table1 G1)
     (on table2 G3)
     (platform table1)
     (platform table2)
     (manipulable ball)
     (agent R)
     })

(def state2
  '#{(at R G2)
     (holds R nil)
     })

drop-on {:pre ((agent ?agent)
                    (at ?agent ?place)
                    (on ?platform ?place)
                    (on nil ?platform)
                    (:not (holds ?agent nil))
                    (holds ?agent ?obj))

             :add ((holds ?agent nil)
                    (on ?obj ?platform))
             :del ((holds ?agent ?obj))
             :txt ((drop ?obj on ?platform))
             :cmd [drop ?obj on ?platform]
             }