  A situation calculus is presented that provides a solution to the frame
problem for hierarchical situations, that is, situations that have a modular
structure in which parts of the situation behave in a relatively independent
manner. This situation calculus is given in a relational, functional, and modal
logic form. Each form permits both a single level hierarchy or a multiple level
hierarchy, giving six versions of the formalism in all, and a number of
sub-versions of these. For multiple level hierarchies, it is possible to give
equations between parts of the situation to impose additional structure on the
problem. This approach is compared to others in the literature.
