“…in [4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42]. For recent progress in F-theory compactifications where one or more U(1) factors are formed, see, for example, [4,7,8,10,43,12,44,45,46,47,17,48,20,...…”