<s>
A	O
two-level	B-Language
grammar	I-Language
is	O
a	O
formal	O
grammar	O
that	O
is	O
used	O
to	O
generate	O
another	O
formal	O
grammar	O
,	O
such	O
as	O
one	O
with	O
an	O
infinite	O
rule	O
set	O
.	O
</s>
<s>
This	O
is	O
how	O
a	O
Van	B-Application
Wijngaarden	I-Application
grammar	I-Application
was	O
used	O
to	O
specify	O
Algol	B-Language
68	I-Language
.	O
</s>
<s>
This	O
makes	O
such	O
two-level	B-Language
grammars	I-Language
more	O
powerful	O
than	O
a	O
single	O
layer	O
of	O
context-free	O
grammar	O
,	O
because	O
generative	O
two-level	B-Language
grammars	I-Language
have	O
actually	O
been	O
shown	O
to	O
be	O
Turing	B-Algorithm
complete	I-Algorithm
.	O
</s>
<s>
Two-level	B-Language
grammar	I-Language
can	O
also	O
refer	O
to	O
a	O
formal	O
grammar	O
for	O
a	O
two-level	O
formal	O
language	O
,	O
which	O
is	O
a	O
formal	O
language	O
specified	O
at	O
two	O
levels	O
,	O
for	O
example	O
,	O
the	O
levels	O
of	O
words	O
and	O
sentences	O
.	O
</s>
