[namespace.future]
Change: New reserved namespaces.
Rationale: Reserve namespaces for future revisions of the standard library
that might otherwise be incompatible with existing programs.
Effect on original feature:
The global namespaces std
followed by an arbitrary sequence of digits
is reserved for future standardization.
Valid C++ 2014 code that uses such a top-level namespace,
e.g., std2, may be invalid in this International Standard.