code: public WorkUnitStream transform(Function<WorkUnit, WorkUnit> function) {
      if (this.materializedWorkUnits == null) {
        return new BasicWorkUnitStream(this, Iterators.transform(this.workUnits, function), null);
      } else {
        return new BasicWorkUnitStream(this, null, Lists.newArrayList(Lists.transform(this.materializedWorkUnits, function)));
      }
    }
docs: Apply a transformation function to this stream.