code: public O setParallelism(int parallelism) {
  		Preconditions.checkArgument(parallelism > 0 || parallelism == ExecutionConfig.PARALLELISM_DEFAULT,
  				"The parallelism must be at least one, or ExecutionConfig.PARALLELISM_DEFAULT (use system default).");
  
  		this.parallelism = parallelism;
  
  		@SuppressWarnings("unchecked")
  		O returnType = (O) this;
  		return returnType;
  	}
docs: Sets the parallelism for this operator.
  The parallelism must be 1 or more.
  
  @param parallelism The parallelism for this operator. A value equal to {@link ExecutionConfig#PARALLELISM_DEFAULT}
  will use the system default.
  @return The operator with set parallelism.