[antlr-interest] semantics of the => operator

[Christian]

Hi community,

I stumble across the => operator and the following rule:

arule
  : (identifier generic_argument_list) => identifier generic_argument_list
  | identifier
  ;

Why is this rule not equivalent to the following one?

brule
  : identifier generic_argument_list?
  ;

I admit that I do not find the operator on the antlr site, thus I do not
read any documentation about the semantics of =>.

Please, can someone tell me?

Thanks,
Christian