[antlr-interest] better error messages in tree parsers

[Terence Parr]

Also note that use use the decision number (24 here) by using -dfa option on antlr and then loading your grammar-dec-24.dot into Graphviz. look at state 3 and you'll see that the token ALT (in this case) has no path to take.
Ter
> ASTVerifier.g: node from after line 150:17 [grammarSpec, rules, rule, altListAsBlock, altList, alternative, elements, element, ebnf, block, altList, alternative]  no viable alt; token=[@-1,0:0='ALT',<84>,0:-1] (decision=24 state 3) decision=<<>> 
> context=...DOWN BLOCK DOWN >>>ALT<<< DOWN DOC_COMMENT...
> 
> Here's my code:
> 
>    public String getErrorMessage(RecognitionException e,
>                                  String[] tokenNames)
>    {
>        List stack = getRuleInvocationStack(e, this.getClass().getName());
>        String msg = null;
>        String inputContext =
>            ((Tree)input.LT(-3)).getText()+" "+
>            ((Tree)input.LT(-2)).getText()+" "+
>            ((Tree)input.LT(-1)).getText()+" >>>"+
>            ((Tree)input.LT(1)).getText()+"<<< "+
>            ((Tree)input.LT(2)).getText()+" "+
>            ((Tree)input.LT(3)).getText();
>        if ( e instanceof NoViableAltException ) {
>           NoViableAltException nvae = (NoViableAltException)e;
>           msg = " no viable alt; token="+e.token+
>              " (decision="+nvae.decisionNumber+
>              " state "+nvae.stateNumber+")"+
>              " decision=<<"+nvae.grammarDecisionDescription+">>";
>        }
>        else {
>           msg = super.getErrorMessage(e, tokenNames);
>        }
>        return stack+" "+msg+" context=..."+inputContext+"...";
>    }
>    public String getTokenErrorDisplay(Token t) {
>        return t.toString();
>    }
> 
> Ter
> 
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