title: How to toggle letter cases in a region in emacs
body: <p>How can I toggle the case of letters (switch uppercase letters to lowercase and lowercase letters to uppercase) of a region's text in Emacs?</p>
  
  <p>There are listed commands for conversion but nothing for toggling.</p>
  
  <p><strong>Example:</strong></p>
  
  <p>PLease toggLE MY LETTER case</p>
  
  <p>should become:</p>
  
  <p>plEASE TOGGle my letter CASE</p>