fix undefined symbols problem

[hiamandeep]

pm is used to show plus-minus, is yet not implemented into sympy. #5305
Example latex: \frac{-b\pm\sqrt{b^2-4ac}}{2a}
when converted into sympy using latex2sympy, I get
-b*pm*sqrt(-4*a*c + b**2)/(2*a)

So, the undefined pm simply gets multiplied. Hence getting wrong expression.
It should have probably shown an error.

In fact, any undefined thing simply gets multiplied
Ex: -b*pm*blabla*(-4*a*c + b**2)/(2*a)

[augustt198]

Maybe \pm should be blacklisted as an invalid symbol because it produces such an unexpected result.

In regards to \blabla or something like that, I don't think it's a good idea to have a whitelist of valid symbols because it would be hard to account for the variety of symbols that people would want to use.