code: public void setParallelism(int parallelism) {
  		if (parallelism < 1 && parallelism != ExecutionConfig.PARALLELISM_DEFAULT) {
  			throw new IllegalArgumentException("Parallelism of " + parallelism + " is invalid.");
  		}
  		this.parallelism = parallelism;
  	}
docs: Sets the parallelism for this optimizer node.
  The parallelism denotes how many parallel instances of the operator will be
  spawned during the execution.
  
  @param parallelism The parallelism to set. If this value is {@link ExecutionConfig#PARALLELISM_DEFAULT}
  then the system will take the default number of parallel instances.
  @throws IllegalArgumentException If the parallelism is smaller than one.