[antlr-interest] What does " The following alternatives can never be matched" mean

[qinglangee]

Gavin Lambert wrote:
> 
> The problem is that both the function and var rules have a common 
> left prefix (Letter+); the only distinguishing feature is whether 
> it's followed by a comma or not.  So when you're saying that 
> "command" can consist of either "function" or "var", it's not 
> possible to define static lookahead to disambiguate that case.
> 
> You can either force explicit lookahead (this doesn't always 
> work):
>    command : ( (function) => function
>              | var
>              )+
>            ;
> 
> Or refactor the rules to eliminate the ambiguity:
>    command : ( Letter+ ','? )+ ;
> 
> Note however that since you're hiding whitespace in the lexer, the 
> input "HELLO" and "HE L      LO" will be treated as 
> identical.  This may or may not be what you want.
> 
> 
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> 

Thanks for your kindness.
I am freshman on antlr. And I donot very clear about the syntax of antlr
lexer,
could sb. advise some document, like the useage of => in the advice above.
The only useful thing I can find is the 
http://www.antlr.org/wiki/display/ST/Five+minute+Introduction Five minute
Introduction .

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