Protecting variables



Hi,

We plan to write a specialized toolbox for people working
in control theory.  As a complete beginner in maxima I 
have a simple question: how can I "protect" a variable 
from being assigned accidentally?  (under maple it is 
done using the protect/unprotect commands).  

Example: if a user type

t:5;

this yields an error and lets "t" unassigned

Thanks in advance for any hint on this question
A. Marquez