<s>
A	O
superimposed	B-General_Concept
code	I-General_Concept
such	O
as	O
Zatocoding	B-General_Concept
is	O
a	O
kind	O
of	O
hash	B-Error_Name
code	I-Error_Name
that	O
was	O
popular	O
in	O
marginal	O
punched-card	O
systems	O
.	O
</s>
<s>
edge-notched	O
cards	O
,	O
slotted	O
cards	O
,	O
E-Z	O
Sort	O
,	O
Zatocards	B-General_Concept
,	O
McBee	O
,	O
McBee	O
Keysort	O
,	O
Flexisort	O
,	O
Velom	O
,	O
Rocket	O
,	O
etc	O
.	O
</s>
<s>
Some	O
sets	O
of	O
cards	O
contained	O
all	O
the	O
information	O
required	O
by	O
the	O
user	O
on	O
the	O
card	O
itself	O
,	O
handwritten	O
,	O
typewritten	O
,	O
or	O
on	O
microfilm	O
(	O
aperture	B-Application
card	I-Application
)	O
.	O
</s>
<s>
The	O
"	O
superimposed	B-General_Concept
code	I-General_Concept
"	O
coding	O
systems	O
,	O
such	O
as	O
Zatocoding	B-General_Concept
,	O
saved	O
space	O
by	O
entering	O
several	O
or	O
all	O
subjects	O
in	O
the	O
same	O
field	O
;	O
such	O
a	O
"	O
superimposed	B-General_Concept
code	I-General_Concept
"	O
stores	O
much	O
more	O
information	O
in	O
less	O
space	O
,	O
but	O
at	O
the	O
cost	O
of	O
occasional	O
"	O
false	O
"	O
selections	O
.	O
</s>
<s>
superimposed	B-General_Concept
code	I-General_Concept
systems	O
can	O
solve	O
this	O
problem	O
.	O
</s>
<s>
The	O
Zatocoding	B-General_Concept
system	O
of	O
information	O
retrieval	O
was	O
developed	O
by	O
Calvin	O
Mooers	O
in	O
1947	O
.	O
</s>
<s>
Calvin	O
Mooers	O
invented	O
Zatocoding	B-General_Concept
at	O
M.I.T.	O
,	O
a	O
mechanical	O
information	O
retrieval	O
system	O
based	O
on	O
superimposed	B-General_Concept
codes	I-General_Concept
,	O
and	O
formed	O
the	O
Zator	B-General_Concept
Company	O
in	O
1947	O
to	O
commercialize	O
its	O
applications	O
.	O
</s>
<s>
The	O
particular	O
superimposed	B-General_Concept
code	I-General_Concept
used	O
in	O
that	O
system	O
is	O
called	O
Zatocoding	B-General_Concept
,	O
</s>
<s>
while	O
the	O
marginal-punched	O
card	O
information	O
retrieval	O
system	O
as	O
a	O
whole	O
is	O
called	O
"	O
Zator	B-General_Concept
"	O
.	O
</s>
<s>
Setting	O
up	O
a	O
superimposed	B-General_Concept
code	I-General_Concept
for	O
a	O
particular	O
library	O
goes	O
something	O
like	O
this	O
:	O
</s>
<s>
(	O
If	O
N	O
>=	O
R	O
,	O
then	O
we	O
could	O
use	O
the	O
"	O
obvious	O
way	O
"	O
mentioned	O
above	O
—	O
the	O
whole	O
point	O
of	O
Zatocoding	B-General_Concept
is	O
that	O
it	O
works	O
even	O
when	O
N	O
is	O
much	O
less	O
than	O
R	O
)	O
.	O
</s>
<s>
Rather	O
than	O
slotting	O
one	O
hole	O
per	O
subject	O
in	O
"	O
the	O
obvious	O
way	O
"	O
,	O
a	O
superimposed	B-General_Concept
code	I-General_Concept
will	O
slot	O
n	O
holes	O
per	O
subject	O
.	O
</s>
<s>
(	O
There	O
are	O
several	O
ways	O
to	O
pick	O
these	O
patterns	O
—	O
those	O
distinguish	O
between	O
the	O
various	O
superimposed	B-General_Concept
codes	I-General_Concept
;	O
we	O
discuss	O
them	O
below	O
)	O
.	O
</s>
<s>
(	O
Several	O
variations	O
of	O
Zatocoding	B-General_Concept
were	O
developed	O
.	O
</s>
<s>
Setting	O
up	O
a	O
Zatocode	B-General_Concept
for	O
a	O
particular	O
list	O
of	O
R	O
subjects	O
goes	O
something	O
like	O
this	O
:	O
</s>
<s>
A	O
Zatocode	B-General_Concept
requires	O
a	O
code	O
book	O
that	O
lists	O
every	O
subject	O
and	O
a	O
randomly	O
generated	O
notch	O
code	O
associated	O
with	O
each	O
one	O
.	O
</s>
<s>
have	O
a	O
fixed	O
hash	B-Error_Name
function	I-Error_Name
for	O
transforming	O
the	O
letters	O
in	O
(	O
one	O
spelling	O
of	O
)	O
a	O
subject	O
into	O
a	O
notch	O
code	O
.	O
</s>
<s>
A	O
Bloom	B-Algorithm
filter	I-Algorithm
can	O
be	O
considered	O
a	O
kind	O
of	O
superimposed	B-General_Concept
code	I-General_Concept
.	O
</s>
