code: @Nonnull
    public final ConcurrentCollectorMultiple <DATATYPE> setPerformer (@Nonnull final IConcurrentPerformer <List <DATATYPE>> aPerformer)
    {
      if (m_aPerformer != null)
        throw new IllegalStateException ("Another performer is already set!");
      m_aPerformer = ValueEnforcer.notNull (aPerformer, "Performer");
      return this;
    }
docs: Set the performer to be used. This method must be invoked before the
  collector can be run. The passed implementation must be rock-solid as this
  class will not make any retries. If the passed performer throws and
  exception without handling the objects correct the objects will be lost!
  
  @param aPerformer
  The performer to be used. May not be <code>null</code>.
  @return this for chaining
  @throws IllegalStateException
  If another performer is already present!