Fence Post Error

 

The computer science term “fence post error” refers to a conceptual mistake that puts you off by one. It’s named after a common fencing miscalculation. Let’s say you’re building a 30-yard fence and you need one post for every yard of fence. How many posts do you need? At first blush, the answer seems like it should be 30 posts. However, unless the fence circles back on itself, you actually need 31. The conceptual error that Zinn, Abrams, and I had made was a fence post error. We were off by one!

Beneath the ninth row from the top

Of the wall including small bricks

I found this very confusing. The word beneath is used here to determine a row for counting, not the location of the casque! Of course the casque is going to be beneath the ninth row, somewhere. Everything in the garden is beneath the ninth row of the wall! It is the horizontal counting, to ten, that is meant to indicate the location of the casque. Therefore, in order to count horizontally we must first count vertically, counting down from the top to identify the proper row and then right to left to find the casque. I know it seems like a knit picky detail, one that we shouldn’t have to notice, but I think that’s the point. That’s what makes it so difficult. 

“Beneath” the ninth row is the tenth, so that’s the row we want to use when counting horizontally, not the ninth!  As I counted downward, again and again, I realized that I arrived at a different row when counting from the front (east) side versus the back (west) side of the wall. From the front side, I arrived at the row that Zinn and Abrams had used for counting horizontally. But the tenth row on the back side was underground! What I’d missed was that the front side has a ledge consisting of “small bricks,” a ledge that doesn’t exist on the backside! The only way to count ten rows of stones vertically is to do so from the front!

The phrase “including small bricks” was a hint. I, like Zinn and Abrams, assumed that it referred to the small bricks on the top row only. It also refers to the small bricks that form the ledge in the front.

Welcome to Level 3.