Yet another proof of Hawking and Ellis's Lemma 8.5.5

Abstract

The fact that the null generators of a future Cauchy horizon are past complete was proved first by Hawking and Ellis [1]. Then Budzy\'nski, Kondracki, and Kr\'olak outlined a proof free from an error found in the original one [2]. Finally, a week ago Minguzzi published his version of proof [3] patching a previously unnoticed hole in the preceding two. I am not aware of any flaws in that last proof, but it is quite difficult. In this note I present a simpler one.