title: RxSwift keep state of string without a Subject or a Variable
body: <p>I want to convert a ViewModel that I already have to be full Rx.
  The application evaluates arithmetic expressions.</p>
  
  <p>The ViewModel has two strings, one with 'logical' symbols, that will be send to the expression evaluator to get a result, and a 'presentation' string with other symbols that will be showed in the UI.</p>
  
  <p>The ViewController calls a VM function, 'keyPressed(keyID: Int)' that will be used to modify both the 'logical' string, and the 'presentation' string, the 'keyPressed' uses the right symbol for the right string and modify it by appending the new symbol.</p>
  
  <p>How can I convert the VM to be reactive without having any state (any string) or at least not having the presentation string?</p>