func isLetter(_ c :Character) ->Bool{
let unicodeValue = c.unicodeScalars.first!.value
if (unicodeValue > 64 && unicodeValue<91) || (unicodeValue > 96 && unicodeValue < 123){
return true
}
return false
}
func tokenLize (_ string:String)->Set<String>{
var a = ""
var b = Set<String>()
for data in string{
if isLetter(data){
a.insert(data, at: a.endIndex)
a = a.lowercased()
}else{
b.insert(a)
a = ""
continue
}
}
return b
}