DAF DISCUSSIONS - ERUVIN 98

S asks:

Abaye answers (just before second Mishna 98b) that we are talking about a large ziz (ledge) which is under a smaller ziz of less than 4 tefochim - hence a mokom petur.

How can there be a mekom petur in the domain of a reshus hayochid (which continues 'ad hooray) ?

S,