<s>
In	O
competitive	O
two-player	O
games	O
,	O
the	O
killer	B-Algorithm
heuristic	I-Algorithm
is	O
a	O
move-ordering	O
method	O
based	O
on	O
the	O
observation	O
that	O
a	O
strong	O
move	O
or	O
small	O
set	O
of	O
such	O
moves	O
in	O
a	O
particular	O
position	O
may	O
be	O
equally	O
strong	O
in	O
similar	O
positions	O
at	O
the	O
same	O
move	O
(	O
ply	O
)	O
in	O
the	O
game	O
tree	O
.	O
</s>
<s>
This	O
technique	O
improves	O
the	O
efficiency	O
of	O
alpha	B-Algorithm
–	I-Algorithm
beta	I-Algorithm
pruning	I-Algorithm
,	O
which	O
in	O
turn	O
improves	O
the	O
efficiency	O
of	O
the	O
minimax	B-Algorithm
algorithm	I-Algorithm
.	O
</s>
<s>
Alpha	B-Algorithm
–	I-Algorithm
beta	I-Algorithm
pruning	I-Algorithm
works	O
best	O
when	O
the	O
best	O
moves	O
are	O
considered	O
first	O
.	O
</s>
<s>
The	O
killer	B-Algorithm
heuristic	I-Algorithm
attempts	O
to	O
produce	O
a	O
cutoff	O
by	O
assuming	O
that	O
a	O
move	O
that	O
produced	O
a	O
cutoff	O
in	O
another	O
branch	O
of	O
the	O
game	O
tree	O
at	O
the	O
same	O
depth	O
is	O
likely	O
to	O
produce	O
a	O
cutoff	O
in	O
the	O
present	O
position	O
,	O
that	O
is	O
to	O
say	O
that	O
a	O
move	O
that	O
was	O
a	O
very	O
good	O
move	O
from	O
a	O
different	O
(	O
but	O
possibly	O
similar	O
)	O
position	O
might	O
also	O
be	O
a	O
good	O
move	O
in	O
the	O
present	O
position	O
.	O
</s>
<s>
By	O
trying	O
the	O
killer	B-Algorithm
move	I-Algorithm
before	O
other	O
moves	O
,	O
a	O
game-playing	O
program	O
can	O
often	O
produce	O
an	O
early	O
cutoff	O
,	O
saving	O
itself	O
the	O
effort	O
of	O
considering	O
or	O
even	O
generating	O
all	O
legal	O
moves	O
from	O
a	O
position	O
.	O
</s>
<s>
In	O
practical	O
implementation	O
,	O
game-playing	O
programs	O
frequently	O
keep	O
track	O
of	O
two	O
killer	B-Algorithm
moves	I-Algorithm
for	O
each	O
depth	O
of	O
the	O
game	O
tree	O
(	O
greater	O
than	O
depth	O
of	O
1	O
)	O
and	O
see	O
if	O
either	O
of	O
these	O
moves	O
,	O
if	O
legal	O
,	O
produces	O
a	O
cutoff	O
before	O
the	O
program	O
generates	O
and	O
considers	O
the	O
rest	O
of	O
the	O
possible	O
moves	O
.	O
</s>
<s>
If	O
a	O
non-killer	O
move	O
produces	O
a	O
cutoff	O
,	O
it	O
replaces	O
one	O
of	O
the	O
two	O
killer	B-Algorithm
moves	I-Algorithm
at	O
its	O
depth	O
.	O
</s>
<s>
A	O
generalization	O
of	O
the	O
killer	B-Algorithm
heuristic	I-Algorithm
is	O
the	O
history	O
heuristic	O
.	O
</s>
