import java.util.HashSet;
/*
 * PredicateSet defines some logical predicates
 */
public class PredicateSet extends HashSet implements Predicate{
  
  /**
   * of(x) is true if x is an element
   * @param x possible element.
   * @return true iff x is an element.
   */
  public boolean of(Object x) {
    return contains(x);
  }

}