code: public FaunusPipeline back(final String step) {
          this.state.assertNotLocked();
          this.state.assertNoProperty();
  
          this.compiler.addMapReduce(BackFilterMapReduce.Map.class,
                  BackFilterMapReduce.Combiner.class,
                  BackFilterMapReduce.Reduce.class,
                  LongWritable.class,
                  Holder.class,
                  NullWritable.class,
                  FaunusVertex.class,
                  BackFilterMapReduce.createConfiguration(this.state.getElementType(), this.state.getStep(step)));
          makeMapReduceString(BackFilterMapReduce.class, step);
          return this;
      }
docs: Go back to an element a named step ago.
  Currently only backing up to vertices is supported.
  
  @param step the name of the step to back up to
  @return the extended FaunusPipeline