  In a recent paper (Phys. Rev. Lett. 109, 160501 (2012). arXiv:1201.0849), it
is claimed that any quantum protocol for classical two-sided computation
between Alice and Bob can be proven completely insecure for Alice if it is
secure against Bob. Here we show that the proof is not sufficiently general,
because the security definition it based on is only a sufficient condition but
not a necessary condition.
