We present a formalism for logic program cooperation based on the answer set semantics. The system consists of independent logic programs that are connected via a sequential communication channel. When presented with an input set of literals from its predecessor, a logic program computes its output as an answer set of itself, enriched with the input. It turns out that the communication strategy makes the system quite expressive: essentially a sequence of a fixed number of programs n captures the complexity class ${\ensuremath{\Sigma}^P_n}$, i.e. the n-th level of the polynomial hierarchy. On the other hand, unbounded sequences capture the polynomial hierarchy $\mathcal{PH}$. These results make the formalism suitable for complex applications such as hierarchical decision making and preference-based diagnosis on ordered theories. In addition, such systems can be realized by implementing an appropriate control strategy on top of existing solvers such as dlv or smodels, possibly in a distributed environment.