title: turn for in loops local variables into mutable variables
body: <p>I've this code that I made on playground to represent my problem:</p>
  
  <pre><code>import Foundation
  
  var countries = ["Poland":["Warsaw":"foo"],"England":["London":"foo"]]
  
  for (country, city) in countries {
    if city["London"] != nil {
     city["London"] = "Piccadilly Circus" // error because by default the variables [country and city] are constants (let)
    }
  } 
  </code></pre>
  
  <p>Does anyone know a work around or the best way to make this work?</p>