String+KeystrokeSubsequences.swift

extension String {

   /// For "TEST" returns ["T", "TE", "TES", "TEST"]
   var keystrokeSubsequences: [String] {
       let start = distance(from: startIndex, to: startIndex)
       let end = distance(from: startIndex, to: endIndex)
       return (start...end).map { index in
           let substringEnd = self.index(startIndex, offsetBy: index)
           let substring = self[..<substringEnd]
           return String(substring)
       }
   }

}