std::pow implementation

[Robert Dewar]

> I do insist on the fact that it is not just an "as-if" rule.

You can insist all you like, but every rule in a standard of this kind
is an as-if rule. It would require a formal apparatus and set of definitions
far beyond what the C++ standard attempts for it to be otherwise.

The standard describes the meaning of a program in formal semantic terms
by giving a formal semantics to C++ programs. It does not specify what
code should be generated, only what that code must mean when it is executed.