More than enough "Always succeed"? [ RAKU ] - grammar

Question

In the documentation of Grammars under Section :
"Always succeed" assertion
I reproduced the example presented there, with added code to show the table produced, in each stage of the parsing mechanism :
use v6.d;
grammar Digifier {
rule TOP { [ <.succ> <digit>+ ]+ }
token succ { <?> }
token digit { <[0..9]> }
}
class Letters {
has #!numbers;
method digit ($/) { #!numbers.tail ~= <a b c d e f g h i j>[$/]; say '---> ' ~ #!numbers }
method succ ($/) { #!numbers.push: '!'; say #!numbers }
method TOP ($/) { make #!numbers[^(*-1)] }
}
say 'FINAL ====> ' ~ Digifier.parse('123 456 789', actions => Letters.new).made;
The result is the following:
[!]
---> !b
---> !bc
---> !bcd
[!bcd !]
---> !bcd !e
---> !bcd !ef
---> !bcd !efg
[!bcd !efg !]
---> !bcd !efg !h
---> !bcd !efg !hi
---> !bcd !efg !hij
[!bcd !efg !hij !]
FINAL ====> !bcd !efg !hij
I expected only 3 pushes in table #!numbers but I got 4.
I am puzzled about the need for an exclusion of the last value of table #!numbers in method "TOP".
Yes I know that the code produces the correct result but why?
Where does the last "Always Succeed" assertion come from?

Answer 1

A quantified group, eg [ A B ]+, is effectively a loop, repeatedly attempting to match A, and iff that matches, attempting to match B.
Unless it's an infinite loop -- in which case your program will hang -- it will eventually match N times and then move on. If A always matches, but your program doesn't hang, then that must mean B eventually failed. If so, A is guaranteed to have matched one more time than B.
In your code, A is <.succ> which reduces to <?>, which always matches. There are 4 attempts and thus A matches 4 times. In contrast B, which is <digit>+, fails on the fourth attempt so only matches 3 times.