code: public O setParallelism(int dop) {
  		if(dop < 1) {
  			throw new IllegalArgumentException("The parallelism of an operator must be at least 1.");
  		}
  		this.dop = dop;
  		
  		@SuppressWarnings("unchecked")
  		O returnType = (O) this;
  		return returnType;
  	}
docs: Sets the degree of parallelism for this operator.
  The degree must be 1 or more.
  
  @param dop The degree of parallelism for this operator.
  @return The operator with set degree of parallelism.