title: Xtext: Boolean feature consuming a token series
body: <p>as you know, it is possible to make a boolean feature like</p>
  
  <pre><code>tokenExist ?= 'TOKEN'?
  </code></pre>
  
  <p>How can i know consume more than one token like this:</p>
  
  <pre><code>tokensExist ?= ('TOKENONE' 'TOKENTWO')?
  </code></pre>
  
  <p>This is not valid in Xtext.</p>
  
  <p>My current solution approach looks like this:</p>
  
  <pre><code>(tokensExist ?='TOKENONE' 'TOKENTWO')?
  </code></pre>
  
  <p>works, but does not really express what is meant</p>
  
  <p>Any ideas?</p>