PDDL2.1 - The Art of the Possible? Commentary on Fox and Long
  PDDL2.1 was designed to push the envelope of what planning algorithms can do,
and it has succeeded. It adds two important features: durative actions,which
take time (and may have continuous effects); and objective functions for
measuring the quality of plans. The concept of durative actions is flawed; and
the treatment of their semantics reveals too strong an attachment to the way
many contemporary planners work. Future PDDL innovators should focus on
producing a clean semantics for additions to the language, and let planner
implementers worry about coupling their algorithms to problems expressed in the
latest version of the language.
