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
In the City Ballet Company any dancer who has danced in Giselle has also danced in Sleeping Beauty, and some dancers who have danced in Sleeping Beauty have also danced in Swan Lake. Ttherefore, some dancers in the City Ballet Company who have danced in Giselle have also danced in Swan Lake.