<s>
Quiescence	B-General_Concept
search	I-General_Concept
is	O
an	O
algorithm	O
typically	O
used	O
to	O
extend	O
search	O
at	O
unstable	O
nodes	O
in	O
minimax	B-Algorithm
game	O
trees	O
in	O
game-playing	O
computer	B-Application
programs	I-Application
.	O
</s>
<s>
It	O
is	O
an	O
extension	O
of	O
the	O
evaluation	B-General_Concept
function	I-General_Concept
to	O
defer	O
evaluation	O
until	O
the	O
position	O
is	O
stable	O
enough	O
to	O
be	O
evaluated	O
statically	O
,	O
that	O
is	O
,	O
without	O
considering	O
the	O
history	O
of	O
the	O
position	O
or	O
future	O
moves	O
from	O
the	O
position	O
.	O
</s>
<s>
It	O
mitigates	O
the	O
effect	O
of	O
the	O
horizon	B-General_Concept
problem	I-General_Concept
faced	O
by	O
AI	B-Application
engines	O
for	O
various	O
games	O
like	O
chess	B-Application
and	O
Go	O
.	O
</s>
<s>
A	O
quiescence	B-General_Concept
search	I-General_Concept
attempts	O
to	O
emulate	O
this	O
behavior	O
by	O
instructing	O
a	O
computer	O
to	O
search	O
"	O
volatile	O
"	O
positions	O
to	O
a	O
greater	O
depth	O
than	O
"	O
quiet	O
"	O
ones	O
to	O
make	O
sure	O
there	O
are	O
no	O
hidden	O
traps	O
and	O
to	O
get	O
a	O
better	O
estimate	O
of	O
its	O
value	O
.	O
</s>
<s>
One	O
common	O
criterion	O
is	O
that	O
moves	O
exist	O
in	O
the	O
position	O
that	O
can	O
dramatically	O
change	O
the	O
valuation	O
of	O
the	O
position	O
,	O
such	O
as	O
captures	O
in	O
chess	B-Application
or	O
Go	O
.	O
</s>
<s>
As	O
the	O
main	O
motive	O
of	O
quiescence	B-General_Concept
search	I-General_Concept
is	O
to	O
get	O
a	O
stable	O
value	O
out	O
of	O
a	O
static	B-General_Concept
evaluation	I-General_Concept
function	I-General_Concept
,	O
it	O
may	O
also	O
make	O
sense	O
to	O
detect	O
wide	O
fluctuations	O
in	O
values	O
returned	O
by	O
a	O
simple	O
heuristic	O
evaluator	O
over	O
several	O
ply	O
,	O
i.e.	O
</s>
<s>
The	O
quiescence	B-General_Concept
search	I-General_Concept
continues	O
as	O
long	O
as	O
the	O
position	O
remains	O
volatile	O
according	O
to	O
the	O
criterion	O
.	O
</s>
<s>
In	O
order	O
to	O
get	O
the	O
quiescence	B-General_Concept
search	I-General_Concept
to	O
terminate	O
,	O
plies	O
are	O
usually	O
restricted	O
to	O
moves	O
that	O
deal	O
directly	O
with	O
the	O
threat	O
,	O
such	O
as	O
moves	O
that	O
capture	O
and	O
recapture	O
(	O
often	O
called	O
a	O
'	O
capture	O
search	O
 '	O
)	O
in	O
chess	B-Application
.	O
</s>
<s>
The	O
horizon	B-General_Concept
effect	I-General_Concept
is	O
a	O
problem	O
in	O
artificial	B-Application
intelligence	I-Application
which	O
can	O
occur	O
when	O
all	O
moves	O
from	O
a	O
given	O
node	O
in	O
a	O
game	O
tree	O
are	O
searched	O
to	O
a	O
fixed	O
depth	O
.	O
</s>
<s>
Quiescence	B-General_Concept
search	I-General_Concept
attempts	O
to	O
mitigate	O
this	O
issue	O
by	O
extending	O
the	O
search	O
depth	O
in	O
volatile	O
positions	O
where	O
the	O
heuristic	O
value	O
may	O
have	O
significant	O
fluctuations	O
between	O
moves	O
.	O
</s>
<s>
This	O
pseudocode	B-Language
illustrates	O
the	O
concept	O
algorithmically	O
:	O
</s>
