[antlr-interest] What's the difference between these two rules?

[chain one]

I am reading the <<The Definitive ANTLR Reference. Building Domain-Specific
Languages>> book
And I am confused about something talked in p315 of Chapter 12.

Could someone tell me what the difference is between the following two
rules?

stat
options{backtrack=true}
: declaration
| expression
;


stat
:(declaration)=> declaration
| expression
;

Thanks for your help.
Best Regards,

Young
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