  We study computation of a modulo-sum of two binary source sequences over a
two-user erasure multiple access channel. The channel is modeled as a
binary-input, erasure multiple access channel, which can be in one of three
states - either the channel output is a modulo-sum of the two input symbols, or
the channel output equals the input symbol on the first link and an erasure on
the second link, or vice versa. The associated state sequence is independent
and identically distributed. We develop a new upper bound on the sum-rate by
revealing only part of the state sequence to the transmitters. Our coding
scheme is based on the compute and forward and the decode and forward
techniques. When a (strictly) causal feedback of the channel state is available
to the encoders, we show that the modulo-sum capacity is increased. Extensions
to the case of lossy reconstruction of the modulo-sum and to channels involving
additional states are also treated briefly.
