code: @SuppressWarnings("unchecked")
      public void series(final Outcome outcome, final Function... functions) {
          _series(null, outcome, functions);  // generic signature problem, hence null
      }
docs: Run an array of functions in series, each one running once the previous function has completed.
  If any functions in the series pass an error to its callback,
  no more functions are run and outcome for the series is immediately called with the value of the error.