<s>
In	O
computer	B-General_Concept
science	I-General_Concept
,	O
more	O
particular	O
in	O
the	O
theory	O
of	O
formal	O
languages	O
,	O
a	O
counter	B-Application
automaton	I-Application
,	O
or	O
counter	O
machine	O
,	O
is	O
a	O
pushdown	B-Application
automaton	I-Application
with	O
only	O
two	O
symbols	O
,	O
and	O
the	O
initial	O
symbol	O
in	O
,	O
the	O
finite	O
set	O
of	O
stack	O
symbols	O
.	O
</s>
<s>
Equivalently	O
,	O
a	O
counter	B-Application
automaton	I-Application
is	O
a	O
nondeterministic	B-General_Concept
finite	I-General_Concept
automaton	I-General_Concept
with	O
an	O
additional	O
memory	O
cell	O
that	O
can	O
hold	O
one	O
nonnegative	O
integer	O
number	O
(	O
of	O
unlimited	O
size	O
)	O
,	O
which	O
can	O
be	O
incremented	O
,	O
decremented	O
,	O
and	O
tested	O
for	O
being	O
zero	O
.	O
</s>
<s>
The	O
class	O
of	O
counter	O
automata	O
can	O
recognize	O
a	O
proper	O
superset	O
of	O
the	O
regular	B-General_Concept
and	O
a	O
subset	O
of	O
the	O
deterministic	O
context	O
free	O
languages	O
.	O
</s>
<s>
For	O
example	O
,	O
the	O
language	O
is	O
a	O
non-regular	O
language	O
accepted	O
by	O
a	O
counter	B-Application
automaton	I-Application
:	O
It	O
can	O
use	O
the	O
symbol	O
to	O
count	O
the	O
number	O
of	O
s	O
in	O
a	O
given	O
input	O
string	O
(	O
writing	O
an	O
for	O
each	O
in	O
)	O
,	O
after	O
that	O
,	O
it	O
can	O
delete	O
an	O
for	O
each	O
in	O
.	O
</s>
<s>
A	O
two-counter	O
automaton	O
,	O
that	O
is	O
,	O
a	O
two-stack	O
Turing	B-Architecture
machine	I-Architecture
with	O
a	O
two-symbol	O
alphabet	O
,	O
can	O
simulate	O
an	O
arbitrary	O
Turing	B-Architecture
machine	I-Architecture
.	O
</s>
