code: public DeltaIteration<ST, WT> parallelism(int parallelism) {
  		Preconditions.checkArgument(parallelism > 0 || parallelism == ExecutionConfig.PARALLELISM_DEFAULT,
  			"The parallelism must be positive, or ExecutionConfig.PARALLELISM_DEFAULT (use default).");
  		this.parallelism = parallelism;
  		return this;
  	}
docs: Sets the parallelism for the iteration.
  
  @param parallelism The parallelism.
  @return The iteration object, for function call chaining.