<s>
PL/0	B-Language
is	O
a	O
programming	O
language	O
,	O
intended	O
as	O
an	O
educational	B-Language
programming	I-Language
language	I-Language
,	O
that	O
is	O
similar	O
to	O
but	O
much	O
simpler	O
than	O
Pascal	B-Application
,	O
a	O
general-purpose	B-Language
programming	I-Language
language	I-Language
.	O
</s>
<s>
It	O
serves	O
as	O
an	O
example	O
of	O
how	O
to	O
construct	O
a	O
compiler	B-Language
.	O
</s>
<s>
While	O
these	O
limitations	O
make	O
writing	O
real	O
applications	O
in	O
this	O
language	O
impractical	O
,	O
it	O
helps	O
the	O
compiler	B-Language
remain	O
compact	O
and	O
simple	O
.	O
</s>
<s>
In	O
the	O
original	O
implementation	O
presented	O
by	O
Wirth	O
,	O
there	O
are	O
no	O
input	B-General_Concept
and	I-General_Concept
output	I-General_Concept
routines	O
.	O
</s>
<s>
The	O
new	O
value	O
of	O
each	O
variable	O
is	O
printed	O
compiler	B-Language
prints	O
as	O
it	O
changes	O
.	O
</s>
<s>
Flow	O
control	O
structures	O
are	O
if-then	B-Language
and	O
while-do	O
constructs	O
and	O
user-defined	O
procedures	O
.	O
</s>
<s>
It	O
is	O
rather	O
easy	O
for	O
students	O
to	O
write	O
a	O
recursive	B-Application
descent	I-Application
parser	I-Application
for	O
such	O
a	O
simple	O
syntax	O
.	O
</s>
<s>
Therefore	O
,	O
the	O
PL/0	B-Language
compiler	B-Language
is	O
still	O
widely	O
used	O
in	O
courses	O
on	O
compiler	B-Language
construction	O
throughout	O
the	O
world	O
.	O
</s>
<s>
Due	O
to	O
the	O
lack	O
of	O
features	O
in	O
the	O
original	O
specification	O
,	O
students	O
usually	O
spend	O
most	O
of	O
their	O
time	O
with	O
extending	O
the	O
language	O
and	O
their	O
compiler	B-Language
.	O
</s>
<s>
The	O
main	O
article	O
on	O
compilers	B-Language
honours	O
PL/0	B-Language
for	O
introducing	O
several	O
influential	O
concepts	O
(	O
stepwise	O
refinement	O
,	O
recursive	B-Application
descent	I-Application
parsing	I-Application
,	O
EBNF	O
,	O
P-code	O
,	O
T-diagrams	O
)	O
to	O
the	O
field	O
by	O
educating	O
students	O
to	O
use	O
these	O
concepts	O
.	O
</s>
<s>
Over	O
the	O
last	O
3	O
decades	O
,	O
most	O
university	O
courses	O
on	O
compiler	B-Language
construction	O
that	O
used	O
PL/0	B-Language
have	O
followed	O
Wirth	O
strictly	O
in	O
employing	O
these	O
techniques	O
(	O
see	O
references	O
below	O
)	O
.	O
</s>
<s>
Some	O
years	O
ago	O
university	O
courses	O
deviated	O
from	O
the	O
course	O
set	O
by	O
Wirth	O
with	O
the	O
replacement	O
of	O
the	O
classical	O
recursive	B-Application
descent	I-Application
parsing	I-Application
technique	O
by	O
a	O
(	O
nonetheless	O
classical	O
)	O
Unix-like	O
approach	O
of	O
employing	O
lex	B-General_Concept
and	O
yacc	B-Application
.	O
</s>
<s>
Only	O
recently	O
an	O
implementation	O
(	O
)	O
along	O
this	O
way	O
has	O
also	O
combined	O
modern	O
concepts	O
like	O
object-orientation	O
and	O
design	O
patterns	O
with	O
a	O
modern	O
scripting	O
language	O
(	O
Python	B-Language
)	O
,	O
allowing	O
students	O
to	O
consume	O
the	O
source	O
text	O
of	O
the	O
implementation	O
in	O
a	O
contemporary	O
programming	O
style	O
.	O
</s>
<s>
In	O
December	O
1976	O
,	O
Wirth	O
wrote	O
a	O
small	O
booklet	O
about	O
compiler	B-Language
construction	O
,	O
containing	O
the	O
full	O
source	O
code	O
of	O
the	O
PL/0	B-Language
compiler	B-Language
.	O
</s>
<s>
In	O
later	O
editions	O
of	O
this	O
book	O
(	O
under	O
the	O
influence	O
of	O
his	O
ongoing	O
research	O
)	O
Wirth	O
changed	O
the	O
syntax	O
of	O
PL/0	B-Language
.	O
</s>
<s>
This	O
change	O
made	O
PL/0	B-Language
resemble	O
Modula-2	B-Language
more	O
closely	O
.	O
</s>
<s>
Wirth	O
added	O
both	O
symbols	O
to	O
the	O
PL/0	B-Language
language	O
,	O
but	O
he	O
did	O
not	O
mention	O
their	O
semantics	O
in	O
the	O
book	O
.	O
</s>
<s>
Most	O
courses	O
in	O
compiler	B-Language
construction	O
today	O
have	O
replaced	O
the	O
exclamation	O
mark	O
with	O
the	O
WriteLn	O
procedure	O
.	O
</s>
<s>
In	O
the	O
third	O
and	O
last	O
edition	O
of	O
his	O
book	O
on	O
compiler	B-Language
construction	O
,	O
Wirth	O
replaced	O
PL/0	B-Language
with	O
Oberon-0	B-Language
.	O
</s>
<s>
The	O
language	O
Oberon-0	B-Language
is	O
much	O
more	O
complex	O
than	O
PL/0	B-Language
.	O
</s>
<s>
For	O
example	O
,	O
Oberon-0	B-Language
offers	O
arrays	O
,	O
records	O
,	O
type	O
declarations	O
and	O
procedure	O
parameters	O
.	O
</s>
