  In this paper we investigate a problem associated with operating a robotic
mobile fulfilment system (RMFS). This is the problem of allocating orders and
mobile storage racks to pickers.
  We present a two-stage formulation of the problem. In our two-stage approach
we, in the first-stage, deal with the orders which must be definitely fulfilled
(picked), where the racks chosen to fulfil these first-stage orders are chosen
so as to (collectively) contain sufficient product to satisfy all orders. In
the second-stage we restrict attention to those racks chosen in the first-stage
solution in terms of allocating second-stage orders.
  We present three different strategies for first-stage order selection; one of
these strategies minimises the requirement to make decisions as to the rack
sequence (i.e. the sequence in which racks are presented to each picker).
  We present a heuristic procedure to reduce the number of racks that need to
be considered. Extensive computational results are presented for test problems
that are made publicly available; including test problems that are
significantly larger than previous problems considered in the literature.
