In other words, "G" is calculated in 64 steps: the first step is to calculate "g"1 with four up-arrows between 3s; the second step is to calculate "g"2 with "g"1 up-arrows between 3s; the third step is to calculate "g"3 with "g"2 up-arrows between 3s; and so on, until finally calculating "G" = "g"64 with "g"63 up-arrows between 3s.
In other words, "G" is calculated in 64 steps: the first step is to calculate "g"1 with four up-arrows between 3s; the second step is to calculate "g"2 with "g"1 up-arrows between 3s; the third step is to calculate "g"3 with "g"2 up-arrows between 3s; and so on, until finally calculating "G" = "g"64 with "g"63 up-arrows between 3s.