Randomised controlled trial of population screening for atrial fibrillation in people aged 70 years and over to reduce stroke: protocol for the SAFER trial by Mant, J, Modi, RN, Dymond, A, Armstrong, N, Burt, J, Calvert, P, Cowie, M, Ding, WY, Edwards, D, Freedman, B, Griffin, SJ, Hoare, S, Hobbs, FDR, Johnson, R, Kaptoge, S, Lip, GYH, Lobban, T, Lown, M, Lund, J, McManus, RJ, Mills, MT, Morris, S, Powell, A, Proietti, R