title: Logic equivalent for string comparison
body: <p>Why are these expressions different:</p>
  
  <pre><code>!x == "string"
  </code></pre>
  
  <p>and</p>
  
  <pre><code>x != "string"
  </code></pre>
  
  <p>At least first expression does not enter my <code>if</code>, and when I change it, <code>if</code> code is executed.</p>