  We give a structure theorem for all coalitionally strategy-proof social
choice functions whose range is a subset of cardinality two of a given larger
set of alternatives.
  We provide this in the case where the voters/agents are allowed to express
indifference and the domain consists of profiles of preferences over a society
of arbitrary cardinality. The theorem, that takes the form of a representation
formula, can be used to construct all functions under consideration.
