<s>
In	O
computer	B-General_Concept
science	I-General_Concept
,	O
particularly	O
in	O
functional	B-Language
programming	I-Language
,	O
hash	B-Application
consing	I-Application
is	O
a	O
technique	O
used	O
to	O
share	O
values	O
that	O
are	O
structurally	O
equal	O
.	O
</s>
<s>
The	O
term	O
hash	B-Application
consing	I-Application
originates	O
from	O
implementations	O
of	O
Lisp	B-Language
that	O
attempt	O
to	O
reuse	O
cons	B-Protocol
cells	O
that	O
have	O
been	O
constructed	O
before	O
,	O
avoiding	O
the	O
penalty	O
of	O
memory	B-General_Concept
allocation	I-General_Concept
.	O
</s>
<s>
Hash	B-Application
consing	I-Application
is	O
most	O
commonly	O
implemented	O
with	O
hash	B-Algorithm
tables	I-Algorithm
storing	O
weak	B-General_Concept
references	I-General_Concept
that	O
may	O
be	O
garbage-collected	B-General_Concept
when	O
the	O
data	O
stored	O
therein	O
contains	O
no	O
references	O
from	O
outside	O
the	O
table	O
.	O
</s>
<s>
Hash	B-Application
consing	I-Application
has	O
been	O
shown	O
to	O
give	O
dramatic	O
performance	O
improvementsboth	O
space	O
and	O
timefor	O
symbolic	B-Algorithm
and	O
dynamic	B-Algorithm
programming	I-Algorithm
algorithms	O
.	O
</s>
<s>
An	O
interesting	O
property	O
of	O
hash	B-Application
consing	I-Application
is	O
that	O
two	O
structures	O
can	O
be	O
tested	O
for	O
equality	O
in	O
constant	O
time	O
,	O
which	O
in	O
turn	O
can	O
improve	O
efficiency	O
of	O
divide	B-Algorithm
and	I-Algorithm
conquer	I-Algorithm
algorithms	I-Algorithm
when	O
data	O
sets	O
contain	O
overlapping	O
blocks	O
.	O
</s>
<s>
Simple	O
,	O
not	O
very	O
efficient	O
,	O
but	O
suitable	O
for	O
demonstration	O
of	O
the	O
concept	O
implementation	O
of	O
a	O
memoizer	O
by	O
means	O
of	O
hash	B-Algorithm
table	I-Algorithm
and	O
weak	B-General_Concept
references	I-General_Concept
in	O
Scheme	B-Language
:	O
</s>
