 This rule has an untypical form in that one of its premises is the denial of a nonmonotonic implication; such rules are called “non-Horn”. If RMon\(_{\pipsim}\) is added to P, then CM\(_{\pipsim}\) can be replaced by the rule that infers \(A\wedge B\pipsim \bot\) from \(A\pipsim \bot\). Arló-Costa and Shapiro (1992) proved that the system obtained by dropping CMon\(_{\pipsim}\) from R is sound and complete with respect to “rough” ranked models in which the ordering of the states is no longer subject to the smoothness condition.