 For instance: imagine experimenter \(S'\) measures the spin of the electron \(S\), and writes the value of this spin on a piece of paper. In principle, experimenter \(S''\) can devise an experiment where she can detect an effect due to interference between the two branches where the spin of the electron (and the text) have one or the other value. But if \(S''\) measures the spin and reads the piece of paper, she will find that experimenter \(S'\) has seen the same spin as herself.