title: label x assigned to a block which is not a set
body: <p>trying out upgrading antlr4, I have 2 lines in the grammar that produce the error message:</p>
  
  <blockquote>
    <p>label tok assigned to a block which is not a set</p>
  </blockquote>
  
  <p>Specifically for a grammar line that looks like this:</p>
  
  <pre><code>contextRadius: tok=('radius' 'change-authorize-nas-ip') (IP4_ADDRESS|IP6_ADDRESS) 'encrypted' 'key' ID 'port' INT_TOK 'event-timestamp-window' INT_TOK 'no-reverse-path-forward-check'
      ;
  </code></pre>
  
  <p>What does this imply, exactly - to be a "block which is not set" and is there a general solution? </p>