  We reconsider the model of quantum mechanics violation in the $\ko$--$\kob$
system, due to Ellis, Hagelin, Nanopoulos, and Srednicki, in which \cp- and
\cpt-violating signatures arise from the evolution of pure states into mixed
states. We present a formalism for computing time-dependent asymmetries in this
model and show that present data constrains its parameters significantly. In
the future, this model will be put to very stringent tests at a $\phi$ factory.
We present the theory of these tests and show the relation between particular
$\phi$ decay correlations and the parameters of quantum mechanics violation.
