code: public void setBroadcastVariable(String name, Operator<?> root) {
  		if (name == null) {
  			throw new IllegalArgumentException("The broadcast input name may not be null.");
  		}
  		if (root == null) {
  			throw new IllegalArgumentException("The broadcast input root operator may not be null.");
  		}
  		
  		this.broadcastInputs.put(name, root);
  	}
docs: Binds the result produced by a plan rooted at {@code root} to a variable
  used by the UDF wrapped in this operator.
  
  @param root The root of the plan producing this input.