code: public T getIncoming(O label) {
          return incomingTransitions.computeIfAbsent(label, k -> listSuppllier.get());
      }
docs: Retrieves the list of incoming transitions for the respective label.
  <p>
  This method will always return a non-{@code null} value.
  
  @param label
  the label
  
  @return the (possibly empty) list associated with the given state label