This is just a verification of Insights No. 2

http://dafyomi.co.il/eruvin/insites/ev-dt-078.htm

(b) RASHI (77b, DH she'Shelivosav) does not accept this reasoning. He maintains that a ledge diminishes the height of a wall even when the ledge is four Tefachim in only one dimension. How, then, does Rashi answer our question?

I looked at this Rashi several times on 77b near the bottom of the Amud. The ledge is on the next Amud. In fact this Rashi doesn't seem to

mention anything about dimensions, unless it's connected to the next one which is discussing a ladder.

Moshe Usher

PS I found both Insights very informative, helpful, and enjoyable.