MMap "access" keyword, choice of exception class

[Tim Peters]

[Tim]
> SystemError is used for things that "can't happen" ...
> For example, if the Python Virtual Machine sees an undefined 
> bytecode; it's akin to an assert.

[Paul Rubin]
> Note that user code can create undefined bytecode, e.g. by
> demarshalling something that looks like a valid code object but isn't.
> So I hope SystemError isn't like an assert in the sense that bytecode
> validity checking is disabled when the debugging flags are turned off.

No, it isn't -- we use "assert" when we want "assert" <wink>.