import java.lang.*;
import java.util.*;

/**
   Interface for a state space that can be traversed by applying 
   operators.
*/

public interface Traversable {

/** Return state obtained by applying op.  null if op is not valid here. */
   public State applyOperator(String op);

/** Return cost of applying op. */
   public float costOf(String op);

/** Get all operators valid from this state. */
   public ArrayList<String> validOperators();
    
}