assigna value to assumed parameter

Hi,

I have this problem when I use assume and assign a value to an assumed parameter.

Example:

restart:assume(beta>0):

g:=int(Sum(5*sin(n*beta*x),n=1..5),x) ;

beta:=1;

g;

Normaly beta:=1 is replaced into g. The result obtained is function of beta and beta:=1 is not replaced.

Why beta:=1 is not replaced in g ?

Thanks for suggestion