But what if that was not the original intention?
R. Nahman laid down in the name of Samuel: If a karpaf that was bigger than two beth se'ah was not originally enclosed for dwelling purposes, how is one to proceed?
The answer: make a breech in the surrounding wall bigger than 10 cubits (so as to make it invalid) and then rebuild that hole into a 10 cubit or small entrance. In effect, rebuilding the wall as if new.
The question is then asked: could then same thing be done by breaking down smaller sections of the fence, say one cubit at a time, and rebuilding it until you’ve reached 10 cubits?
The answer is given by analogy. With a sandal.
A sandal becomes ritually defiled by treading on something – a dead body, a reptile, etc. That defilement (midras – from the root daras, “to tread”) stays with the sandal and defiles everything it touches – human beings and vessels. Tearing off a strap and then repairing that strap does not change it. But tearing off a second strap and repairing it does. Why? Because the appearance has now changed – i.e. it looks like a new sandal.
The same is true of the fence. It looks new, so it is treated as new.