Goal Stacks

I)	II)
On(a,b)	On(b,c)
On(b,c)	On(a,b)
On(a,b) and On(b,c)	On(a,b) and On(b,c)

consider: multiple conjunctions ... permutations

*each time!

Look at this table from bottom up (remember stack)

On(C,A)	Operator Sequence
Clear(C)	(Plan)
ArmEmpty
Clear(C) and ArmEmpty and On(C,A)	Unstack(C,A)
-Unstack (C,A)	PutDown(C)
ArmEmpty	PickUp(A)
Clear(A) and ArmEmpty and OnTable(A)	Stack(A,B)
-PickUp (A)	.
Clear(B) and Holding (A)	.
-Stack (A,B)	.
On (B,C)
On (A,B) and On(B,C)	picture now
->	world view

On(B,C)

On(A,B) and On(B,C)

.

.

.

->

On(A,B) and On(B,C)

->

On(A,B)

On(A,B) and On(B,C)

Need: method capable of finding efficient way!

linear - no concept of 'ease' ... which first?

additional processing (repair): where perform operation then immediately undo

Complete operation stack

0. UnStack(C,A)
1. PutDown (C)
2. PickUp (A)
3. Stack (A,B)
4. UnStack(A,B)
5. PutDown(A)
6. PickUp(B)
7. Stack (B,C)
8. PickUp(A)
9. Stack (A,B)

wasted p.s. efforts
produce steps

undo (eliminate)

complex tasks
many alternatives

reduction difficult

(interfering operations)

Solution:

do not plan sequentially to achieve compound goals (i.e., consider effects)

nonlinear planning

E.g., hierarchical (degree of difficulty) opportunistic