code: public FaunusPipeline filter(final String closure) {
          this.state.assertNotLocked();
          this.state.assertNoProperty();
  
          this.compiler.addMap(FilterMap.Map.class,
                  NullWritable.class,
                  FaunusVertex.class,
                  FilterMap.createConfiguration(this.state.getElementType(), this.validateClosure(closure)));
          makeMapReduceString(FilterMap.class);
          return this;
      }
docs: Emit or deny the current element based upon the provided boolean-based closure.
  
  @param closure return true to emit and false to remove.
  @return the extended FaunusPipeline