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C00002 00002	%glasgo.sli[e89,jmc]	Slide for unexpected obstacle problem
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%glasgo.sli[e89,jmc]	Slide for unexpected obstacle problem
\input slide2.tex[1,jmc]
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\overfullrule=0pt
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$$succeeds(a,s) ≡ (∀p)(precond(p,a) ⊃ holds(p,s))$$
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$$succeeds(a,s) ∧ causes(a,p) ⊃ holds(p,result(a,s))$$
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$$¬noninertial(p,a) ∧ holds(p,s) ⊃ holds(p,result(a,s))$$
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$$occurs(e,s) ⊃ outcome s = outcome result(e,s)$$
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$$∀e¬occurs(e,s) ⊃ outcome s = s$$
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$$rr(a,s) = outcome result(e,s)$$

$$causes(fly(x,y),at y)$$
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$$precond(at x,fly(x,y))$$
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$$precond(hasticket,fly(x,y))$$
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$$precond(existsflight(x,y),fly(x,y))$$

$$causes(loseticket, not hasticket)$$
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$$causes(buyticket,hasticket)$$
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$$holds(not p,s) ≡ ¬holds(p,s)$$

$$holds(at Glasgow,S0)$$
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$$holds(hasticket,S0)$$
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$$holds(existsflight(Glasgow,London),S0)$$
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$$holds(existsflight(London,Moscow),S0)$$

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$$circum(Facts;causes,precond,noninertial,occurs;holds)$$

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We can show
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$$\eqalign{at(Moscow,rr&(fly(London,Moscow),\cr
&rr(fly(Glasgow,London),S0))),\cr}$$
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but not if we add
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$$occurs(loseticket,result(fly(Glasgow,London),S0)).$$
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However, in this case we can show
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$$\eqalign{at(Moscow,rr&(fly(London,Moscow),\cr
&rr(buyticket,\cr
&\ \ rr(fly(Glasgow,London),S0)))).\cr}$$
\vfill\end