title: Having one input field auto update another, and vice versa (probably using JavaScript)
body: <p>I'm not that experienced in JavaScript, so need som help with a project.</p>
  
  <p>I have a form of, among other things, 3 input text fields (let's call them A, B and C). I need the user to only be able to type into either A+B <em>OR</em> C. So is it possible to make a form field inactive, once the user clicks in another (and vice versa).</p>
  
  <p>Would really appreciate any help. My guess is this needs t be done in JavaScript.</p>