[harerazer]

I'm clearly misunderstanding something here. Assuming the rest of the board is empty, if e1 is occupied, the rook can't move to e1, whereas if it isn't occupied, the rook can move to e1, so there is at least one position in which we must check e1. What am I misunderstanding here?

[Jasper_]

Oh, I was assuming it was occupied by an enemy piece, and would capture it. You're right that if it was occupied by an allied piece, it's not a legal move.