title: 4.1 parser claims "mismatched input" with simple palindrome grammar
body: <p>Can't see my mistake here. Given a simple palindrome grammar:</p>
  
  <pre><code>// test1.g
  
  grammar test1;
  
  start
      : 'a' start 'a'
      | 'b' start 'b'
      | 'a' 
      | 'b'
      |
      ;
  
  WS : [ \t\r\n]+ -&gt; skip ;
  </code></pre>
  
  <p>when given the input <code>aabbbaa</code>, the generated parser reports:</p>
  
  <pre><code>line 1:5 mismatched input 'a' expecting 'b'
  </code></pre>
  
  <p>But it looks like legal input to me. Antlr v4.1.</p>