Rabbah's Gezeirah – Eruvin 33a
תוספות עירובין דף לג/א ד"ה והא אי בעי מייתי לה דרך עליו
In Meseches Beitzah (17b), the Mishnah rules that it is forbidden to immerse vessels in a mikveh on Shabbos. Rabbah explains that this prohibition is a rabbinic decree, which was enacted by the Rabbis lest a person come to transport the vessel four amos in a reshus ha'rabbim (a public domain). Tosafos (ad loc. d.h. Shema Ya'avirenu) asks why Rabbah is not concerned with the more likely circumstance that the person may come to remove the vessel from his reshus ha'yachid (private domain) to a reshus ha'rabbim? Tosafos answer that there are doorways and fence-posts that remind a person not to carry from a reshus ha'yachid to a reshus ha'rabbim; however, there are no signs in a reshus ha'rabbim to alert a person that he is about to carry an object more than four amos.
Nachalas Yaakov (loc. cit.) suggests a different answer to this question. He understands that our Tosafos teach us that a person who carries from one reshus ha'yachid to another reshus ha'yachid only violates the Torah prohibition of Hotza'ah if he traverses four amos or more of reshus ha'rabbim in between. Without these four amos, a person violates no prohibition, as both the removal (akirah) and placement (hanachah) were done in reshuyos ha'yachid. However, in traversing four amos of reshus ha'rabbim while carrying the vessel, one violates Ma'avir, a derivative (toladahi) of Hotza'ah.
Accordingly, states the Nachalas Yaakov, the question posed by Tosafos in Meseches Beitzah is answered. It is certainly unlikely that a person carrying a vessel from his house to a mikveh will put it down in a public thoroughfare. Hence, realistically, the prohibition under discussion must be that of transporting the vessel from reshus ha'yachid to reshus ha'yachid through reshus ha'rabbim. And, for that prohibition to be violated, the object must be transported at least four amos through a reshus ha'rabbim. It is to those four amos that Rabbah refers. The Nachalas Yaakov adds that this is the way in which the underlying reason for the decrees prohibiting shofar and megillah on Shabbos are to be understood.