<s>
In	O
computer	B-General_Concept
science	I-General_Concept
CDR	B-Language
coding	I-Language
is	O
a	O
compressed	B-General_Concept
data	I-General_Concept
representation	O
for	O
Lisp	B-Language
linked	B-Data_Structure
lists	I-Data_Structure
.	O
</s>
<s>
It	O
was	O
developed	O
and	O
patented	O
by	O
the	O
MIT	O
Artificial	O
Intelligence	O
Laboratory	O
,	O
and	O
implemented	O
in	O
computer	O
hardware	O
in	O
a	O
number	O
of	O
Lisp	B-Operating_System
machines	I-Operating_System
derived	O
from	O
the	O
MIT	B-Operating_System
CADR	I-Operating_System
.	O
</s>
<s>
CDR	B-Language
coding	I-Language
is	O
in	O
fact	O
a	O
fairly	O
general	O
idea	O
;	O
whenever	O
a	O
data	O
object	O
A	O
ends	O
in	O
a	O
reference	O
to	O
another	O
data	O
structure	O
B	O
,	O
we	O
can	O
instead	O
place	O
the	O
structure	O
B	O
itself	O
there	O
,	O
overlapping	O
and	O
running	O
off	O
the	O
end	O
of	O
A	O
.	O
</s>
<s>
By	O
doing	O
this	O
we	O
free	O
the	O
space	O
required	O
by	O
the	O
reference	O
,	O
which	O
can	O
add	O
up	O
if	O
done	O
many	O
times	O
,	O
and	O
also	O
improve	O
locality	B-General_Concept
of	I-General_Concept
reference	I-General_Concept
,	O
enhancing	O
performance	O
on	O
modern	O
machines	O
.	O
</s>
<s>
The	O
transformation	O
is	O
especially	O
effective	O
for	O
the	O
cons-based	O
lists	O
it	O
was	O
created	O
for	O
;	O
we	O
free	O
about	O
half	O
of	O
the	O
space	O
for	O
each	O
node	O
we	O
perform	O
this	O
transformation	O
on	O
.	O
</s>
<s>
In	O
the	O
presence	O
of	O
mutable	B-Application
objects	I-Application
,	O
CDR	B-Language
coding	I-Language
becomes	O
more	O
complex	O
.	O
</s>
<s>
Not	O
only	O
are	O
such	O
moves	O
typically	O
expensive	O
or	O
impossible	O
,	O
but	O
over	O
time	O
they	O
cause	O
fragmentation	B-Architecture
of	O
the	O
store	O
.	O
</s>
<s>
This	O
problem	O
is	O
typically	O
avoided	O
by	O
using	O
CDR	B-Language
coding	I-Language
only	O
on	O
immutable	B-Application
data	O
structures	O
.	O
</s>
