; Output: list of operators or FAILURE
(defun search (state)
  (let (op oplist newstate)
    (if (terminalp state)
        (if (goalp state) '() 'failure)
        (progn
          (setq op (choose-op state))
          (setq newstate
                (funcall op state))
          (setq oplist (search newstate))
          (if (eq oplist 'failure)
              'failure
              (cons op oplist)) ) ) ))

Complications:

• We may have to try different operators.
  ( newstate might be a dead end.)

• It may not be possible to apply op.

• Applying op might violate a constraint.

• We could get into a loop applying op and its inverse.

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