title: RAML : How to require parameter A OR parameter B
body: <p>I'm writing some REST documentation with RAML but I'm stuck.</p>
  
  <p>My problem:
   - I have a GET request used for search that can take a parameter <em>"id"</em> or (<strong>exclusive or</strong>) <em>"reference"</em>. Having <strong>only one of them</strong> is required.</p>
  
  <p>I know how to say "this param is required" but I don't know how to say "having one of these params is required". Is it even possible?</p>