Initialization
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When Soar starts up, it generates and selects the first goal.  Soar's default
knowledge then proposes a top-level problem space and state where the top
level state contains input from the outside environment.  For this example,
we will assume that the top state contains the current configuration of
blocks in the environment.  In this example, block A is on top of B, and
blocks B and C are on the table (see Figure ? on page ?).  The traces of
problem solving show the number of the decision on the left, followed by G:,
P:, S:, or O:.  These signify which context slot has a new value on this
decision.  These are followed by a symbol which is the identifier of the
object selected for that slot.  Following the object identifier is a
simplified representation of the substructure of the object, usually
including its name (such as top-state).  For example, the initial trace looks
as follows, showing that goal G1 is the current goal at decision 0 followed
by selections of the top-level problem space and state.

0   G: G1
1   P: P1 (top-ps)
2   S: S1 (top-state) 
    Initial state has A on B and B and C on the table.
    The goal is to get A on B on C on the table.