Artificial Intelligence A Modern Approach (3rd Edition): page 90.
Bidirectional search.
Note: Based on the description of this algorithm i.e. 'Bidirectional search is implemented by replacing the goal test with a check to see whether the frontiers of the two searches intersect;', it is possible for the searches to pass each other's frontiers by, in particular if the problem is not fully reversible (i.e. unidirectional links on a graph), and could instead intersect at the explored set.
@author Ciaran O'Reilly
@author Mike Stampone
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