European Union (Notification of Withdrawal) Bill

If all this is unnecessary, why was the Prime Minister asked for, and why did she give, a specific undertaking that this matter will be brought before both Houses of Parliament at the end of the process? Surely that shows that in relation to this vital constitutional issue it is not enough to rely on the possibility of the House of Commons exerting its power and, if an undertaking is given, why is it not in the Bill?