<s>
is	O
a	O
header	B-Language
file	I-Language
in	O
the	O
C++	B-Language
Standard	I-Language
Library	I-Language
.	O
</s>
<s>
GCC	B-Application
's	O
implementation	O
declares	O
the	O
namespace	O
(	O
nested	O
within	O
)	O
in	O
the	O
following	O
manner	O
:	O
</s>
<s>
GCC	B-Application
's	O
implementation	O
defines	O
the	O
mechanism	O
as	O
follows	O
.	O
</s>
