I read Lucien Hardy's paper whose tittle was "Quantum Mechanics, Local Realistic Theories, and Lorentz Invariant Relativistic Theories". There, he argued that lorentz invariant observables which involved locality assumption contradict quantum mechanics. I tried to follow his argument, but got stuck on the paragraph right after eq. num. (17). Looks like he mixed outputs from different experiments (replaced/removed beam splitter). I couldn't understand why that was allowed and seemed logic on the paper. So, if anybody gets a better understanding on this, please explain to me.