code: void addCombinableExecutionGraph(DStreamExecutionGraph combinableExecutionGraph){
  		if (this.combinableExecutionGraphs == null){
  			this.combinableExecutionGraphs = new ArrayList<>();
  		}
  		this.combinableExecutionGraphs.add(combinableExecutionGraph);
  	}
docs: Ads the given {@link DStreamExecutionGraph} to the {@link List} of combinable {@link DStreamExecutionGraph}.
  Each added {@link DStreamExecutionGraph} implies some type of
  combine functionality with current operation (i.e., join, union, unionAll etc.)