code: @PublicEvolving
  	public SingleOutputStreamOperator<T> forceNonParallel() {
  		transformation.setParallelism(1);
  		transformation.setMaxParallelism(1);
  		nonParallel = true;
  		return this;
  	}
docs: Sets the parallelism and maximum parallelism of this operator to one.
  And mark this operator cannot set a non-1 degree of parallelism.
  
  @return The operator with only one parallelism.