title: How can I fix the warning: Implicit token definition in parser rule
body: <p>I simplified my Antlr4 grammar to this:</p>
  
  <pre><code>grammar test;
  
  many:
      ('[' INT ']')*;
  INT: '-'? '0'..'9'+;
  </code></pre>
  
  <p>in ANTLRWorks 2.1 it gives me the warning on '[' and ']'</p>
  
  <pre><code> Implicit token definition in parser rule
  </code></pre>
  
  <p>Why they are warnings?</p>
  
  <p>I shouldn't use direct char or string in a parser rule?</p>