There are a few possible Boulder Crab configurations that work here. The simplest one to set up was hinted at by disoriented: If there are no crabs in the way of the rattlesnake at all, it will move north right after you let it free, until it reaches the wall. Afterwards,
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×it would start moving to the west until it reaches
that wall...
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×From there on, it'd move south once and then go east.
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×Unless the way to the east is blocked, of course.
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×