code: public List<DStreamExecutionGraph> getCombinableExecutionGraphs(){
  		return this.combinableExecutionGraphs == null 
  				? Collections.emptyList() 
  						: Collections.unmodifiableList(this.combinableExecutionGraphs);
  	}
docs: Returns the {@link List} of combinable {@link DStreamExecutionGraph}
  where each {@link DStreamExecutionGraph} implies some type of
  combine functionality with current operation (i.e., join, union, unionAll etc.)