CS 641 Lecture -*- Outline -*-

* Preconditions and Guards (11.6)

   These characterize parts of the state space where abortion can't
   occur or where we can choose to make it occur or not, and the same
   for miracles.

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     PRECONDITIONS AND GUARDS

Definition: Let wg.C : S |-> G.
Then m, t: (S |-> G) -> P(S) are defined
by

miracle precondition:
  m.(wg.C)               
    == (\intersect q \in P(G) :: wg.C.q)

abortion guard:
  t.(wg.C) 
    == (\union q \in P(G) :: wg.C.q)

abortion precondition:
  a.(wg.C) == !t.(wg.C)

miracle guard:
  g.(wg.C) == !m.(wg.C)
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        Q: What do these mean intuitively?

        The most useful are the abortion guard, t, and the miracle
        guard, g.

        Q: Why is m.(wg.C) \subseteq t.(wg.C)?

        So these divide the state space as in figure 11.2.
        What are these in terms of the game interpretation?