Chairwoman Tsai Ying-wen of the opposition Democratic Progressive Party (DPP) said yesterday she has asked the United States to openly assured that there has been no change in its policy towards Taiwan.Speaking at an election campaign rally in Yilan County in northeastern Taiwan, Tsai said while meeting American Institute in Taiwan (AIT) Chairman Raymond F. Burghardt the previous day in Taipei, she questioned whether there had been a possible change in U.S. policy on Taiwan.