title: happy: reduce/reduce conflict
body: <p>Why this raises a warning about reduce/reduce conflict</p>
  
  <pre><code>root : set1 'X'     
       | set2 'X' 'X' 
  
  set1 : 'A'          
       | 'B'             
  
  set2 : 'B'          
       | 'C'  
  </code></pre>
  
  <p>but the next is ok?</p>
  
  <pre><code>root : 'A' 'X'     
       | 'B' 'X'     
       | 'B' 'X' 'X'  
       | 'C' 'X' 'X' 
  </code></pre>