<s>
In	O
computer	O
science	O
,	O
a	O
hash	B-Application
tree	I-Application
(	O
or	O
hash	B-Error_Name
trie	B-General_Concept
)	O
is	O
a	O
persistent	B-Application
data	I-Application
structure	I-Application
that	O
can	O
be	O
used	O
to	O
implement	O
sets	O
and	O
maps	B-Application
,	O
intended	O
to	O
replace	O
hash	B-Algorithm
tables	I-Algorithm
in	O
purely	B-Application
functional	I-Application
programming	I-Application
.	O
</s>
<s>
In	O
its	O
basic	O
form	O
,	O
a	O
hash	B-Application
tree	I-Application
stores	O
the	O
hashes	B-Error_Name
of	O
its	O
keys	O
,	O
regarded	O
as	O
strings	O
of	O
bits	O
,	O
in	O
a	O
trie	B-General_Concept
,	O
with	O
the	O
actual	O
keys	O
and	O
(	O
optional	O
)	O
values	O
stored	O
at	O
the	O
trie	B-General_Concept
's	O
"	O
final	O
"	O
nodes	O
.	O
</s>
<s>
Hash	B-Data_Structure
array	I-Data_Structure
mapped	I-Data_Structure
tries	I-Data_Structure
and	O
Ctries	B-Algorithm
are	O
refined	O
versions	O
of	O
this	O
data	O
structure	O
,	O
using	O
particular	O
type	O
of	O
trie	B-General_Concept
implementations	O
.	O
</s>
