  Starting from the Henneaux-Teitelboim action for a chiral scalar, which
generalizes to curved space the Floreanini-Jackiw action, we give two simple
derivations of the exact consistent gravitational anomaly. The first derivation
is through the Schwinger-DeWitt regularization. The second exploits
cohomological methods and uses the fact that in dimension two the
diffeomorphism transformations are described by a single ghost which allows to
climb the cohomological chain in a unique way.
