title: How can redux-form validation based on multiple fields be implemented?
body: <p>Redux Form validation examples only include <a href="https://github.com/erikras/redux-form/tree/v2.4.5#synchronous-validation">individual field validations</a>. </p>
  
  <p>Having a list of a certain number of fields that are the same, how can I model a validation that requires at least one of the inputs to be a valid email address?</p>
  
  <p>For example, for the given form made up of the following fields:</p>
  
  <ul>
  <li>User 1,</li>
  <li>User 2,</li>
  <li>User 3,</li>
  <li>Special User 4</li>
  </ul>
  
  <p>How can I write a validation function that requires Special User 4 to always be set, and either one of the first 3 users?</p>