Is there a way to tell CAS that a variable is a constant?

[webmasterpdx]

I can use assume to specify that a variable is an integer or >0 etc, etc....but is there a way to tell the cas system that a variable is a constant?
I've been having problems with the integration system and most of these problems would be easier for the system to resolve if it knew that certain variables were constants....something like assume(a,constant) (which doesn't work)....

Any ideas?
Thanks in advance
-Donald

[parisse]

I don't understand what constant would mean. For example if a is a free variable then integrating some expressions containing a with respect to x will consider that a is a constant (w.r.t. x). But as explained in another thread, definite integration is harder for expressions with parameters, and you might need assumptions otherwise the system does not know the limit at the endpoints (for example int(exp(-a*x),x,0,inf) does not converge if a<=0).

[webmasterpdx]

By constant I mean the value of the variable with not change throughout the bounds of the function.