  We study mechanism which operate on ordinal preference information (i.e.,
rank ordered lists of alternatives) on the full domain of weak preferences that
admits indifferences. We present a novel decomposition of strategyproofness
into three axioms: separation monotonic, separation upper invariant, and
separation lower invariant. Each axiom is a natural restriction on how
mechanisms can react when agents change their opinion about the relative
ranking of any two adjacently ranked groups of alternatives. Our result extends
a result from (Mennle and Seuken, 2017), a decomposition of strategyproofness
for strict preferences, to the full domain that includes weak preferences.
