code: public void addSuppressedEvent(SuppressedEvent e) {
          if (e == null) {
              throw new IllegalArgumentException("e is null");
          } else if (this.suppressedEvents == null) {
              this.suppressedEvents = new HashSet<>();
          }
          this.suppressedEvents.add(e);
      }
docs: Adds a {@linkplain SuppressedEvent} to the set of suppressed events of
  this Event. If a capable EventProvider determines that a certain event
  has been prevented because its listener class has been registered on this
  event using {@link #preventCascade(Class)}, then the prevented event is
  registered here using this method.
  
  @param e The event to add.
  @throws IllegalArgumentException If the event is <code>null</code>.