"We now consider measurements of A and B when they are compatible observables. Suppose we measure A first and obtain result a'. Subsequently, we may measure B and get result b'. Finally we measure A again. It follows from our measurement formalism that the third measurement always gives a' with certainty; that is, the second (B) measurement does not destroy the previous information obtained in the first (A) measurement. This is rather obvious when the eigenvalues of A are nondegenerate:|\alpha \rangle \xrightarrow{\text{A measurement}} |a', b' \rangle \xrightarrow{\text{B measurement}} |a', b' \rangle \xrightarrow{\text{A measurement}} |a', b' \rangle."