let bom = "\u{FEFF}" // U+FEFF (ZERO WIDTH NO-BREAK SPACE)
bom.count // -> 1
"\(bom)abc".count // -> 4
NSString(string: bom).length // -> 0 (correct: 1)
NSString(string: "\(bom)abc").length // -> 3 (correct: 4)
NSString(string: "\(bom)\(bom)abc").length // -> 4 (correct: 5)
NSString(string: "a\(bom)bc").length // -> 4
NSString(string: "\(bom)234567890123").length // -> 12 (correct: 13)
NSString(string: "\(bom)2345678901234").length // -> 14

let string = "\(bom)abc"
let middleIndex = string.index(string.startIndex, offsetBy: 2)
string.enumerateSubstrings(in: middleIndex..<string.endIndex, options: .byLines) { (_, _, _, _) in }  // <- crash