  Motivated by applications of rateless coding, decision feedback, and ARQ, we
study the problem of universal decoding for unknown channels, in the presence
of an erasure option. Specifically, we harness the competitive minimax
methodology developed in earlier studies, in order to derive a universal
version of Forney's classical erasure/list decoder, which in the erasure case,
optimally trades off between the probability of erasure and the probability of
undetected error. The proposed universal erasure decoder guarantees universal
achievability of a certain fraction $\xi$ of the optimum error exponents of
these probabilities (in a sense to be made precise in the sequel). A
single--letter expression for $\xi$, which depends solely on the coding rate
and the threshold, is provided. The example of the binary symmetric channel is
studied in full detail, and some conclusions are drawn.
