MMap "access" keyword, choice of exception class

[Paul Rubin]

"Tim Peters" <tim.one at home.com> writes:
> SystemError is used for things that "can't happen" -- a SystemError is
> Python telling you that something is insane in Python's own implementation.
> For example, if the Python Virtual Machine sees an undefined bytecode; it's
> akin to an assert.

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.