title: Simple grammar looping infinitely
body: <p>I would expect this simple grammar to match strings such as 'abc':</p>
  <pre><code>grammar Hello;
  
  entry
    :  LETTER+
    ;
  
  LETTER : [a-z] ;
  </code></pre>
  <p>But it seems to enter an infinite loop:</p>
  <pre><code>C:\Code\antlr\hello&gt;antlr4 Hello.g4 -encoding utf8
  
  C:\Code\antlr\hello&gt;javac Hello*.java
  
  C:\Code\antlr\hello&gt;grun Hello entry -tree
  asdf^Z
  
  Terminate batch job (Y/N)? y
  </code></pre>
  <p>Why?</p>