code: public DeltaIteration<ST, WT> parallelism(int parallelism) {
  		Validate.isTrue(parallelism > 0 || parallelism == -1, "The degree of parallelism must be positive, or -1 (use default).");
  		this.parallelism = parallelism;
  		return this;
  	}
docs: Sets the degree of parallelism for the iteration.
  
  @param parallelism The degree of parallelism.
  @return The iteration object, for function call chaining.