= Sussman anomaly
{c}
The <Sussman anomaly> is a <blocks world> example in which independently constructing the obvious subplans for two stacking goals gives destructive <goal interaction in planning>. Clearing a block in advance allows a short joint <plan>, while prematurely achieving either isolated goal can block the other. It demonstrates the inadequacy of restricted <goal-by-goal planning>, rather than a prohibition on every possible ordering of final goal achievements. Preparatory actions can make such an ordering succeed. For a concrete instance, start with $C$ on $A$ and $A,B$ on the table, and require $A$ on $B$ and $B$ on $C$. Moving $C$ to the table, then $B$ onto $C$, then $A$ onto $B$ succeeds. The first move is preparation that an isolated shortest subplan for the second stacking goal would omit.
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