Nonmonotonic Inferences with Qualitative Conditionals based on Preferred
  Structures on Worlds
  A conditional knowledge base R is a set of conditionals of the form "If A,
the usually B". Using structural information derived from the conditionals in
R, we introduce the preferred structure relation on worlds. The preferred
structure relation is the core ingredient of a new inference relation called
system W inference that inductively completes the knowledge given explicitly in
R. We show that system W exhibits desirable inference properties like
satisfying system P and avoiding, in contrast to e.g. system Z, the drowning
problem. It fully captures and strictly extends both system Z and skeptical
c-inference. In contrast to skeptical c-inference, it does not require to solve
a complex constraint satisfaction problem, but is as tractable as system Z.
