title: Xtext DSL: boolean rule with 2 different string
body: <p>Currently I have problem in defining a boolean variable :</p>
  
  <p>I have a class with a boolean variable:</p>
  
  <pre><code>Pet:
     isFeline ?= 'cat' | isFeline ?= 'dog' ;
  </code></pre>
  
  <p>However this result in Pet returning with 'cat'/'dog' as true. Is there anyway to define DSL: 'cat' as true and 'dog' as false ??</p>