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Tail-Recursive Search
The search algorithm can also be written in a tail-recursive style.
- The path (sequence of operators) to the goal is passed down
as a variable, initially '().
- Each time an operator op is applied to create a new state,
op is consed onto the path for the recursive call.
Since cons acts as a stack and makes a list in backwards order,
the path is a backwards list of operators taken from the start.
- When the goal is found, a reverse of the path gives a
solution (sequence of operators to get from the start to the goal).