An air traveler in Beijing cannot fly to Lhasa without first flying to Chengdu  Unfortunately, an air traveler in Beijing must fly to Xian before flying to Chengdu
Any air traveler who flies from Beijing to Lhasa, ttherefore, cannot avoid flying to Xian
A doctor cannot prescribe porozine for a patient without first prescribing anthroxine for that patient. Unfortunately, anthroxine makes most patients who take it feel either extremely drowsy or else extremely nervous. It is likely, ttherefore, that a patient who has taken porozine has felt extremely nervous.