An example task: The Blocks World
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The task we will use is from a Blocks World where the blocks actually exist
in an external environment.  In this task, there is a table and three blocks,
labeled A, B, and C; each block is the same size.  A block is either on the
table or centered on top of another block.  Figure ? shows two examples of
legal configurations of blocks. (The exact locations of the blocks on the
table are irrelevant.) The task is to move the blocks, one at a time, from
some initial configuration to some desired configuration (such as shown in
Figure ?) where the blocks on the table must be in a tower with block A on
top of block B, which is on top of block C, which is on the table. The blocks
can be moved one at a time from the table to the top of a clear block, from
on top of a block to the table, or from on top of a block to a clear block.
We will use the initial configuration where block A is on block B, and blocks
B and C are on the table.