"assume is not additive. Instead, it automatically deletes all previous assumptions on the variables in condition."
Here is a simple test of isequaln and isAlways:
>> syms x y
>> assume(x^2 == y)
>> assumeAlso(x*y == y*x)
>> assumptions
ans =
[ x*y == x*y, x^2 == y]
>> isequaln(x*y,y*x)
ans =
logical
1
% (I'm guessing this is true because multiplication of complex numbers is commutative,
% not because of the above assumption.)
>> isequaln(x*x,y)
ans =
logical
0
>> isAlways(x*x==y)
ans =
logical
1