  PDDL+ is an extension of PDDL2.1 which incorporates fully-featured autonomous
processes and allows for better modelling of mixed discrete-continuous domains.
Unlike PDDL2.1, PDDL+ lacks a logical semantics, relying instead on
state-transitional semantics enriched with hybrid automata semantics for the
continuous states. This complex semantics makes analysis and comparisons to
other action formalisms difficult. In this paper, we propose a natural
extension of Reiter's situation calculus theories inspired by hybrid automata.
The kinship between PDDL+ and hybrid automata allows us to develop a direct
mapping between PDDL+ and situation calculus, thereby supplying PDDL+ with a
logical semantics and the situation calculus with a modern way of representing
autonomous processes. We outline the potential benefits of the mapping by
suggesting a new approach to effective planning in PDDL+.
