next(1,2).
next(2,3).
next(3,4).
next(4,5).
next(5,6).
next(6,7).
next(7,8).
next(8,9).
next(9,10).
next(10,11).
next(11,12).
next(12,13).
next(10,14).
next(14,15).
next(15,16).
next(16,17).
next(17,18).
next(18,19).
next(19,20).
doAt(startflow,2).
doAt(a5,3).
doAt(a13,4).
doAt(a11,5).
doAt(a1,6).
doAt(a2,7).
doAt(a8,8).
doAt(a15,9).
doAt(a3,10).
doAt(a7,11).
doAt(endflow,12).
doAt(a14,14).
doAt(a6,15).
doAt(a10,16).
doAt(a12,17).
doAt(a4,18).
doAt(endflow,19).
leftrightnexts(11,14).