  A planning domain, as any model, is never complete and inevitably makes
assumptions on the environment's dynamic. By allowing the specification of just
one domain model, the knowledge engineer is only able to make one set of
assumptions, and to specify a single objective-goal. Borrowing from work in
Software Engineering, we propose a multi-tier framework for planning that
allows the specification of different sets of assumptions, and of different
corresponding objectives. The framework aims to support the synthesis of
adaptive behavior so as to mitigate the intrinsic risk in any planning modeling
task. After defining the multi-tier planning task and its solution concept, we
show how to solve problem instances by a succinct compilation to a form of
non-deterministic planning. In doing so, our technique justifies the
applicability of planning with both fair and unfair actions, and the need for
more efforts in developing planning systems supporting dual fairness
assumptions.
