Prevent assignments to special names

[olazo]

I'd like to propose that certain special names should be protected so that they could not become variable names (for example pi, e, and i)

if by accident you assign them like:

sage: e=factorial(10)

and then you need to need to use e with it's standard meaning, like

sage: e^100

you will have a very hard time finding the error ( factorial(10)^100 ).

this has been discussed in: ​http://groups.google.com/group/sage-support/browse_thread/thread/b391ed6bd14cdfd0

and

​http://groups.google.com/group/sage-devel/browse_thread/thread/a8e1e4ffe24840dc

[was]

his is something that can (and will) be made an optional mode. We'll have several -- implicit multiplication, automatic variable name creation, and protected names. Having them all on could be useful for people teaching, e.g., calculus labs. This protected mode, by the way, will in the long run use the same technique as automatic variable name creation: replacing the globals() dictionary by a customized wrapper. This has a slight performance penalty, so definitely can't be the default. But it is a fine idea for an optional mode.

[burcin]

I see that this would be a desirable feature for users of symbolics. I don't think the implementation is related to the symbolics subsystem however. I'm assigning this back to the misc component.