  We study logic for reasoning with if-then formulas describing dependencies
between attributes of objects which are observed in consecutive points in time.
We introduce semantic entailment of the formulas, show its fixed-point
characterization, investigate closure properties of model classes, present an
axiomatization and prove its completeness, and investigate alternative
axiomatizations and normalized proofs. We investigate decidability and
complexity issues of the logic and prove that the entailment problem is NP-hard
and belongs to EXPSPACE. We show that by restricting to predictive formulas,
the entailment problem is decidable in pseudo-linear time.
