Often the same position can occur after several differing move sequences,
an example is the Tiger opening which can begin with
f5-d6-c3-d3-c4 or f5-d6-c4-d3-c3, both sequences resulting in
the same position.
This is called a transposition.
To avoid searching a position several times, Zebra
stores most positions it has encountered
during its search in a transposition table (implemented
as the data structure hash table).
For each position, Zebra's assessment and the best move
found during the search is stored.
In the midgame phase of the game, this typically reduces the time
for a deep search by 20 - 50%.
In the endgame, where transpositions are much more common,
the gain can be much higher.