<s>
TXL	B-Language
is	O
a	O
special-purpose	O
programming	O
language	O
originally	O
designed	O
by	O
Charles	O
Halpern-Hamu	O
and	O
James	O
Cordy	O
at	O
the	O
University	O
of	O
Toronto	O
in	O
1985	O
.	O
</s>
<s>
The	O
acronym	O
"	O
TXL	B-Language
"	O
originally	O
stood	O
for	O
"	O
Turing	B-Language
eXtender	O
Language	O
"	O
after	O
the	O
language	O
's	O
original	O
purpose	O
,	O
the	O
specification	O
and	O
rapid	B-Device
prototyping	I-Device
of	O
variants	O
and	O
extensions	O
of	O
the	O
Turing	B-Language
programming	I-Language
language	I-Language
,	O
but	O
no	O
longer	O
has	O
any	O
meaningful	O
interpretation	O
.	O
</s>
<s>
Modern	O
TXL	B-Language
is	O
specifically	O
designed	O
for	O
creating	O
,	O
manipulating	O
and	O
rapidly	O
prototyping	O
language-based	O
descriptions	O
,	O
tools	O
and	O
applications	O
using	O
source	O
transformation	O
.	O
</s>
<s>
It	O
is	O
a	O
hybrid	O
functional	B-Language
/	O
rule-based	B-Language
language	O
using	O
first	O
order	O
functional	B-Language
programming	I-Language
at	O
the	O
higher	O
level	O
and	O
term	O
rewriting	O
at	O
the	O
lower	O
level	O
.	O
</s>
<s>
The	O
formal	B-Application
semantics	I-Application
and	O
implementation	O
of	O
TXL	B-Language
are	O
based	O
on	O
formal	O
term	O
rewriting	O
,	O
but	O
the	O
term	O
structures	O
are	O
largely	O
hidden	O
from	O
the	O
user	O
due	O
to	O
the	O
example-like	O
style	O
of	O
pattern	O
specification	O
.	O
</s>
<s>
Each	O
TXL	B-Language
program	O
has	O
two	O
components	O
:	O
a	O
description	O
of	O
the	O
source	O
structures	O
to	O
be	O
transformed	O
,	O
specified	O
as	O
a	O
(	O
possibly	O
ambiguous	O
)	O
context-free	O
grammar	O
using	O
an	O
extended	O
Backus	O
–	O
Naur	O
Form	O
;	O
and	O
a	O
set	O
of	O
tree	O
transformation	O
rules	O
,	O
specified	O
using	O
pattern	O
/	O
replacement	O
pairs	O
combined	O
using	O
first	O
order	O
functional	B-Language
programming	I-Language
.	O
</s>
<s>
TXL	B-Language
is	O
designed	O
to	O
allow	O
explicit	O
programmer	O
control	O
over	O
the	O
interpretation	O
,	O
application	O
,	O
order	O
and	O
backtracking	O
of	O
both	O
parsing	O
and	O
rewriting	O
rules	O
,	O
allowing	O
for	O
expression	O
of	O
a	O
wide	O
range	O
of	O
grammar-based	O
techniques	O
such	O
as	O
agile	O
parsing	O
.	O
</s>
<s>
The	O
first	O
component	O
parses	O
the	O
input	O
expression	O
into	O
a	O
tree	O
using	O
pattern-matching	B-Language
.	O
</s>
<s>
The	O
second	O
component	O
uses	O
Term-rewriting	O
in	O
a	O
manner	O
similar	O
to	O
Yacc	B-Application
to	O
produce	O
the	O
transformed	O
output	O
.	O
</s>
<s>
TXL	B-Language
is	O
most	O
commonly	O
used	O
in	O
software	O
analysis	O
and	O
reengineering	O
tasks	O
such	O
as	O
design	O
recovery	O
,	O
and	O
in	O
rapid	B-Device
prototyping	I-Device
of	O
new	O
programming	O
languages	O
and	O
dialects	O
.	O
</s>
