- ...table
- The concepts of final
position and tower-deadlock will be
defined formally later on.
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- ...strategy
- If we assume the existence
of an initial situation 9#9, the
problem of determining
what sequences of actions may be selected
according to a particular strategy can be
seen as the problem of determining what
situations are selectable for that strategy.
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- ...4#4
- The situational argument 4#4 in the
predicates Good, Bad and Better
allows reasoning about multiple goals. For
example, by substituting the variable 4#4
by two different constants 5#5 and
6#6 we can express the fact that
performing action a at situation s is
good for the purpose of achieving the goal
described by situation 5#5, but bad for
achieving the goal described by situation
6#6.
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- ...situation
- The idea here is to
select always the best possible action.
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- ...achieved
- We define 10#10
and Prec(a,s) formally later on. In general,
a situation s achieves the
goal described by another situation 4#4 if
all the conditions (propositional fluents)
that hold at 4#4 hold at s
as well. Prec(a,s) is true if action
a can be performed at situation s.
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- ...)
- In the paper, we use the
expression ``propositional fluent'' and
the word ``proposition'' to refer to the
corresponding reified formula.
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- ...symbols
- The symbols h and
g are meta-variables ranging over
distinct function symbols; the expressions
16#16 and 17#17 represent
tuples of variables.
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- ...that
- We use
the following notation to abbreviate the
description of situations 19#19,
and 20#20, where l is
a sequence of actions (i.e., sequences of
actions are applied from left to right).
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- ...values
- The symbols 23#23 and
24#24 are syntactical variables ranging
over block constants.
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- ...30#30)
- Notice
the distinction between 31#31
(axiom ) and reachable
32#32 (axiom ). It is crucial
for understanding the role of the goal
situation 33#33 in the formalization.
The fact that axiom implies
that the goal situation 33#33 is not
reachable from the initial situation 34#34
does not imply that the planning problem is
not solvable. When the program selects an
action (see axiom defining
the fluent selectable), it
tries to find a situation reachable
from the initial situation at which
the goal is achieved. That is, a
situation that satisfies the conditions
imposed on the goal situation. In this sense,
we could say that the role of the goal
situation 33#33 is purely descriptive,
as far as this paper is concerned.
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- ...partially
- Axioms and
are not explicit definitions of
final and above, because these
symbols occur both on the left and
right hand sides. But these
formulas are strong enough for deriving
both positive and negative ground instances
of Holds(Above(x,y),s) and
Holds(Final(x,S36#36),s) from the
positive and negative ground instances of
Holds(On(x,y),s) that can be derived
from axioms to .
[Davis1990] points out that it is possible
to define on in terms of beneath
(37#37),
but it is not possible to fully
define beneath in terms of on in a
first order theory.
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- ...sets
- In the
following, we denote the universal
closure of a formula A by A'.
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- ...logic
- Notice that the only second
order axiom in 41#41 is an axiom of
induction for situations, which we do not
need to decide the selectability of a
single situation.
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- ...descriptions
- The expression
(strategy) can also be
parameterized with respect to the problem
description (axioms and ),
the theory of action (axioms
to ), or the mechanism for action
selection (axiom ).
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- ...strategies
- A
strategy is state-based if whether an
action is good or bad for a given goal at a
particular situation depends only on what
holds at that situation (i.e. the state
associated with that situation), and not on
what situations or actions have been
selected so far. All the strategies
considered in the paper are state-based.
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- ...HREF="node1.html#safe2">
- Strategy-2 is defined, in fact,
by two action selection rules. The first one is
axiom , the second is as follows
57#57.
This rule guarantees that the strategy terminates
as shown in fig. . The rest
of the strategies that terminate do not need this
rule, because it is subsumed by axiom .
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- ...actions
- It
compares the maximum length of the solutions
generated by strategies 4 and 5.
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- ...bad
- Notice that
axiom implies
Better(Move(A,F),Move(A,T),S36#36,S66#66).
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