As formula_37 models the two labs formula_63 and formula_69 within quantum theory, he writes down the state at different times. He knows that the state at time formula_40 of the protocol of system formula_161(i.e., after formula_36' s measurement ) is formula_163 where formula_164denotes agent formula_38 's state of "ready to measure". After the next step of the protocol (formula_38 's measurement at time formula_57 ), this state evolves to formula_168which in turn can be written in the formula_64-basis as formula_170From this, formula_37 can conclude with certainty that formula_38 must have measured formula_91 if formula_37 obtains the outcome formula_76.