code: public FaunusPipeline sideEffect(final String closure) {
          this.state.assertNotLocked();
          this.state.assertNoProperty();
  
          this.compiler.addMap(SideEffectMap.Map.class,
                  NullWritable.class,
                  FaunusVertex.class,
                  SideEffectMap.createConfiguration(this.state.getElementType(), this.validateClosure(closure)));
  
          makeMapReduceString(SideEffectMap.class);
          return this;
      }
docs: Emit the element, but compute some sideeffect in the process.
  For example, mutate the properties of the element.
  
  @param closure the sideeffect closure whose results are ignored.
  @return the extended FaunusPipeline