Effects of active referral combined with a small financial incentive on smoking cessation: study protocol for a cluster randomised controlled trial oleh Man Ping Wang, Tai Hing Lam, Ho Cheung William LI, Yee Tak Derek Cheung, Xue Weng, Ching Yin Lau, Antonio Cho Shing Kwong, Vienna Wai Yin Lai, Sophia Siu Chee Chan