code: public Set<Class<?>> getPrevented() {
          if (this.prevent == null) {
              return Collections.emptySet();
          }
          return Collections.unmodifiableSet(this.prevent);
      }
docs: Gets the listener classes to which dispatching should be prevented while
  this event is being dispatched.
  
  @return The listener classes marked to prevent.
  @see #preventCascade(Class)