Question: Remove Assumptions

I am having trouble removing assumptions that are stored within expresssions.

Example code:

assume(l1>0): # this assumptions later helps to find a solution for a geometric problem with two four-bar-linkages

a := sqrt(l1);

save a, "test.m";

restart;

read "test.m"

a; # the assumptions are stored within the saved data

l1:='l1'; # try to remove the assumption

a; # assumption in a still existing

subs({l1=2}, a); # nothing happens: I can not access l1 any more

subs({l1~=2}, a); # This does not work either, nothing changes in a

So my question is: How do I remove the assumption within a stored expression?

My main problem lies in the handling of the expression with assumptions. At some point, I want to generate Matlab code, and the codegen-command gives me:

Warning, the following variable name replacements were made: l1~ -> cg