  We derive the capacity region of arbitrarily varying multiple-access channels
with conferencing encoders for both deterministic and random coding. For a
complete description it is sufficient that one conferencing capacity is
positive. We obtain a dichotomy: either the channel's deterministic capacity
region is zero or it equals the two-dimensional random coding region. We
determine exactly when either case holds. We also discuss the benefits of
conferencing. We give the example of an AV-MAC which does not achieve any
non-zero rate pair without encoder cooperation, but the two-dimensional random
coding capacity region if conferencing is possible. Unlike compound
multiple-access channels, arbitrarily varying multiple-access channels may
exhibit a discontinuous increase of the capacity region when conferencing in at
least one direction is enabled.
