Let formula_13 be the set of "alternatives" (which is assumed finite), also called "candidates", even if they are not necessarily persons: they can also be several possible decisions about a given issue. We denote by formula_14 the set of "voters". Let formula_15 be the set of strict weak orders over formula_13: an element of this set can represent the preferences of a voter, where a voter may be indifferent regarding the ordering of some alternatives. A "voting rule" is a function formula_17. Its input is a profile of preferences formula_18 and it yields the identity of the winning candidate.