variable in PDDL that can be of more than one type?

Question

Is there any version of this statement that is valid, expressing that an actor knows another actor or a location? Or must I specify separate predicates knows_actor and knows_location?

 (:predicates     
    (knows     ?a - ACTOR ?thing - ACTOR LOCATION)
 )

Answer 1

If the planner you are using supports types, a possible simple solution is to define a supertype in the types section:

ACTOR LOCATON - ACTOR_OR_LOCATION 

and then define the second parameter of the predicate (?thing) with the type ACTOR_OR_LOCATION