  We revisit the derivation of expurgated error exponents using a method of
type class enumeration, which is inspired by statistical-mechanical methods,
and which has already been used in the derivation of random coding exponents in
several other scenarios. We compare our version of the expurgated bound to both
the one by Gallager and the one by Csiszar, Korner and Marton (CKM). For
expurgated ensembles of fixed composition codes over finite alphabets, our
basic expurgated bound coincides with the CKM expurgated bound, which is in
general tighter than Gallager's bound, but with equality for the optimum type
class of codewords. Our method, however, extends beyond fixed composition codes
and beyond finite alphabets, where it is natural to impose input constraints
(e.g., power limitation). In such cases, the CKM expurgated bound may not apply
directly, and our bound is in general tighter than Gallager's bound. In
addition, while both the CKM and the Gallager expurgated bounds are based on
Bhattacharyya bound for bounding the pairwise error probabilities, our bound
allows the more general Chernoff distance measure, thus giving rise to
additional improvement using the Chernoff parameter as a degree of freedom to
be optimized.
