<s>
In	O
computer	B-General_Concept
science	I-General_Concept
,	O
terminal	B-Algorithm
and	I-Algorithm
nonterminal	I-Algorithm
symbols	I-Algorithm
are	O
the	O
lexical	O
elements	O
used	O
in	O
specifying	O
the	O
production	O
rules	O
constituting	O
a	O
formal	O
grammar	O
.	O
</s>
<s>
Nonterminal	B-Algorithm
symbols	O
(	O
or	O
syntactic	O
variables	O
)	O
are	O
replaced	O
by	O
groups	O
of	O
terminal	O
symbols	O
according	O
to	O
the	O
production	O
rules	O
.	O
</s>
<s>
The	O
terminals	O
and	O
nonterminals	B-Algorithm
of	O
a	O
particular	O
grammar	O
are	O
two	O
disjoint	B-Algorithm
sets	I-Algorithm
.	O
</s>
<s>
On	O
the	O
other	O
hand	O
,	O
ר	O
has	O
two	O
rules	O
that	O
can	O
change	O
it	O
,	O
thus	O
it	O
is	O
nonterminal	B-Algorithm
.	O
</s>
<s>
Nonterminal	B-Algorithm
symbols	O
are	O
those	O
symbols	O
that	O
can	O
be	O
replaced	O
.	O
</s>
<s>
A	O
formal	O
grammar	O
includes	O
a	O
start	O
symbol	O
,	O
a	O
designated	O
member	O
of	O
the	O
set	O
of	O
nonterminals	B-Algorithm
from	O
which	O
all	O
the	O
strings	O
in	O
the	O
language	O
may	O
be	O
derived	O
by	O
successive	O
applications	O
of	O
the	O
production	O
rules	O
.	O
</s>
<s>
Context-free	O
grammars	O
are	O
those	O
grammars	O
in	O
which	O
the	O
left-hand	O
side	O
of	O
each	O
production	O
rule	O
consists	O
of	O
only	O
a	O
single	O
nonterminal	B-Algorithm
symbol	O
.	O
</s>
<s>
These	O
are	O
exactly	O
the	O
languages	O
that	O
can	O
be	O
recognized	O
by	O
a	O
non-deterministic	O
push	B-Application
down	I-Application
automaton	I-Application
.	O
</s>
<s>
A	O
grammar	O
is	O
defined	O
by	O
production	O
rules	O
(	O
or	O
just	O
'	O
productions	O
 '	O
)	O
that	O
specify	O
which	O
symbols	O
may	O
replace	O
which	O
other	O
symbols	O
;	O
these	O
rules	O
may	O
be	O
used	O
to	O
generate	B-General_Concept
strings	I-General_Concept
,	O
or	O
to	O
parse	O
them	O
.	O
</s>
<s>
A	O
finite	O
set	O
of	O
nonterminal	B-Algorithm
symbols	O
.	O
</s>
<s>
A	O
finite	O
set	O
of	O
terminal	O
symbols	O
that	O
is	O
disjoint	B-Algorithm
from	O
.	O
</s>
<s>
where	O
is	O
the	O
Kleene	O
star	O
operator	O
and	O
denotes	O
set	O
union	O
,	O
so	O
represents	O
zero	O
or	O
more	O
symbols	O
,	O
and	O
means	O
one	O
nonterminal	B-Algorithm
symbol	O
.	O
</s>
<s>
That	O
is	O
,	O
each	O
production	O
rule	O
maps	O
from	O
one	O
string	O
of	O
symbols	O
to	O
another	O
,	O
where	O
the	O
first	O
string	O
contains	O
at	O
least	O
one	O
nonterminal	B-Algorithm
symbol	O
.	O
</s>
<s>
In	O
this	O
example	O
,	O
the	O
symbols	O
(	O
)	O
are	O
terminal	O
symbols	O
and	O
and	O
are	O
nonterminal	B-Algorithm
symbols	O
.	O
</s>
<s>
In	O
this	O
example	O
,	O
the	O
symbols	O
are	O
terminal	O
symbols	O
and	O
are	O
nonterminal	B-Algorithm
symbols	O
.	O
</s>
