title: cannot create implicit token for string literal in non-combined grammar
body: <p>so found a nice grammar for a calculator and copied it with some lil changes from here:
  <a href="https://dexvis.wordpress.com/2012/11/22/a-tale-of-two-grammars/" rel="noreferrer">https://dexvis.wordpress.com/2012/11/22/a-tale-of-two-grammars/</a></p>
  
  <p>I have two Files: Parser and Lexer. Looks like this: </p>
  
  <pre><code>    parser grammar Parser;
  
  options{
      language = Java;
      tokenVocab = Lexer;
  }
  
  // PARSER
  program : ((assignment|expression) ';')+;
  
  assignment : ID '=' expression;
  
  expression
      : '(' expression ')'                # parenExpression
      | expression ('*'|'/') expression   # multOrDiv
      | expression ('+'|'-') expression   # addOrSubtract
      | 'print' arg (',' arg)*            # print
      | STRING                            # string
      | ID                                # identifier
      | INT                               # integer;
  
  arg : ID|STRING;    
  </code></pre>
  
  <p>and the Lexer:</p>
  
  <pre><code>    lexer grammar WRBLexer;
  
  STRING : '"' (' '..'~')* '"';
  ID     : ('a'..'z'|'A'..'Z')+;
  INT    : '0'..'9'+;
  WS     : [ \t\n\r]+ -&gt; skip ;
  </code></pre>
  
  <p>Basically just splitted Lexer and Parser into two files. 
  But when i try to save i get some Errors: </p>
  
  <pre><code>error(126): Parser.g4:9:35: cannot create implicit token for string literal in non-combined grammar: ';'
  error(126): Parser.g4:11:16: cannot create implicit token for string literal in non-combined grammar: '='
  error(126): Parser.g4:2:13: cannot create implicit token for string literal in non-combined grammar: '('
  error(126): Parser.g4:2:28: cannot create implicit token for string literal in non-combined grammar: ')'
  error(126): Parser.g4:3:10: cannot create implicit token for string literal in non-combined grammar: 'print'
  error(126): Parser.g4:3:23: cannot create implicit token for string literal in non-combined grammar: ','
  error(126): Parser.g4:9:37: cannot create implicit token for string literal in non-combined grammar: '*'
  error(126): Parser.g4:9:41: cannot create implicit token for string literal in non-combined grammar: '/'
  error(126): Parser.g4:10:47: cannot create implicit token for string literal in non-combined grammar: '+'
  error(126): Parser.g4:10:51: cannot create implicit token for string literal in non-combined grammar: '-'
  10 error(s)
  </code></pre>
  
  <p>Hope someone can help me with this. </p>
  
  <p>Best regards</p>