title: ANTLR4 - label assigned to a block which is not a set
body: <p>I am getting the errors <code>label assigned to a block which is not a set</code>. This error occurs for my labels: <code>child</code>, <code>left</code>, <code>right</code>, <code>first</code>, and <code>last</code>. What I am doing is assigning a label to a group of alternatives; shouldn't this be supported?</p>
  
  <p>Fragment of my grammar:</p>
  
  <pre><code>expr:
        unaryExpr '(' child=(stat | expr | NUMBER) ')'                                                #labelUnaryExpr
      | binaryExpr '(' left=(stat | expr | constant) ',' right=(stat | expr | constant) ')'           #labelBinaryExpr
      | multipleExpr '(' first=(stat | expr | constant) (',' rest=(stat | expr | constant))+ ')'      #labelMultipleExpr
      ;   
  </code></pre>