  Hybrid systems are dynamical systems with continuous evolution of states and
discrete evolution of states and governing equations. We have worked on the
design and implementation of HydLa, a constraint-based modeling language for
hybrid systems, with a view to the proper handling of uncertainties and the
integration of simulation and verification. HydLa's constraint hierarchies
facilitate the description of constraints with adequate strength, but its
semantical foundations are not obvious due to the interaction of various
language constructs. This paper gives the declarative semantics of HydLa and
discusses its properties and consequences by means of examples.
