20 Library introduction [library]

20.5 Library-wide requirements [requirements]

20.5.4 Constraints on programs [constraints] Namespace use [namespace.constraints] Namespaces for future standardization [namespace.future]

Top level namespaces with a name starting with std and followed by a non-empty sequence of digits are reserved for future standardization. The behavior of a C++ program is undefined if it adds declarations or definitions to such a namespace. [Example: The top level namespace std2 is reserved for use by future revisions of this International Standard. end example]