title: Swift 3 string insert
body: <p>I have a label in which the text changes, based on the some context I need to insert a number "9" as a string into the labels string. So lets say I have "-102" I want to insert "9" into the string so it turns out like this "-9102", can someone some me how I would code that. The 
        label.insert("9", ind:2)
  doesn't work.</p>