code: @Nonnull
    public final ValidationExecutionManager addExecutor (@Nonnull final IValidationExecutor aExecutor)
    {
      ValueEnforcer.notNull (aExecutor, "Executor");
      m_aExecutors.add (aExecutor);
      return this;
    }
docs: Add a single executor.
  
  @param aExecutor
  The executor to be added. May not be <code>null</code>.
  @return this for chaining
  @see #addExecutors(IValidationExecutor...)
  @see #addExecutors(Iterable)