title: immutable in F#
body: <p>I know that variables in F# are immutable by default.
  But, for example in F# interactive:</p>
  
  <pre><code>  &gt; let x = 4;;
  
  val x : int = 4
  
  &gt; let x = 5;;
  
  val x : int = 5
  
  &gt; x;;
  val it : int = 5
  &gt; 
  </code></pre>
  
  <p>So, I assign 4 to x, then 5 to x and it's changing. Is it correct? Should it give some error or warning? Or I just don't understand how it works?</p>