The TDD(Test-Driven Development) practice for Xcode Swift 4.
Mumbling Kata exercise is from Codewars.
The examples below show you how to write function accum:
accum("abcd") // -> "A-Bb-Ccc-Dddd"
accum("RqaEzty") // -> "R-Qq-Aaa-Eeee-Zzzzz-Tttttt-Yyyyyyy"
accum("cwAt") // -> "C-Ww-Aaa-Tttt"
The parameter of accum is a string which includes only letters from a..z and A..Z.