I know we can use subscript to cut a part of the string in Swift 4, .

```
let s = "aString"
let subS = s[..<s.endIndex]

```

But the problem is, how to cut the `s` to a subString like `aStr`.  

I mean, What I want to do is something like `s[..<(s.endIndex-3)]`.  

But it's not right.  

So, how to do it in Swift 4.