code: @Internal
  	public void addOperator(StreamTransformation<?> transformation) {
  		Preconditions.checkNotNull(transformation, "transformation must not be null.");
  		this.transformations.add(transformation);
  	}
docs: Adds an operator to the list of operators that should be executed when calling
  {@link #execute}.
  
  <p>When calling {@link #execute()} only the operators that where previously added to the list
  are executed.
  
  <p>This is not meant to be used by users. The API methods that create operators must call
  this method.