sorry to bother you but i have a problen in MASECHET SHABAT PAGE 5B:
Ben Azai holds that when someone walks from one Reshus of Chiyuv to another through a domain of Petur (such as a Karmelis), he is Patur, because it is as if he stops with every step. Consequently, it is as if he stopped in the Karmelis, before carrying into the second Reshus, in which case, he will not be Chayav.
ACCORDING TO BEN AZAI IF HE STOPS EVERY STEP - HE WILL NEVER BE CHAYAV IN RESHUT HARABIM!! - I SAW THAT TOS' DISCUSS THIS PROBLEM - BUT I DIDN'T UNDERSTANT IT - COULD YOU HELP ME!
The Tosfos you mention brings a Yerushalmi that says that according to Ben Azai, one will only be Chayav if he jump four Amos. Tosfos says that the Bavli holds that since walking four Amos in Reshus ha'Rabim is a Halachah l'Moshe mi'Sinai, it is an exception to the rule of Mehalech k'Omed.