Swift String Extension with charAt(at: Int) method to return a single char at zero-based int position

StringCharAt.swift

// In the process of creating my first Swift Application using the book "Cocoa Programming for OSX",
// I grumbled at what seemed like clumsy, string-handling, then hit a roadblock that nearly turned me completely off. 
// The string handling code that had worked with Swift 1.2 wouldn't work at all in Swift 3.0.  When I figured out
// what did work in the latest version I was even more annoyed at the verbosity of it.  I could have just left it alone
// and dismissed it as not worth wasting any more of my time, but my curiosity got the best of me and I started looking at
// the language guide on Apple's Developer site.  There I discovered something that totally reversed my thinking: extensions.
// I had also read why string-handling had to be that way (flexibility, multiple Unicode encodings, yada, yada, yada). But 
// I learned that I could utilize extensions to add my own methods to the String class that would give me more convient 
// string-handling for my own needs. So after much long-windedness, here is my charAt method.

extension String {
    // charAt(at:) returns a character at an integer (zero-based) position.
    // example:  
    // let str = "hello"
    // var second = str.charAt(at: 1)
    //  -> "e"  
    func charAt(at: Int) -> Character {
        let charIndex = self.index(self.startIndex, offsetBy: at)
        return self[charIndex]
    }
}