I will change the example to avoid this issue.

It seems summation notation in it's classical form is not up to the demands we place on it these days. I know I'm late to the conversation, but I think the choice of the Karr convention is a bad idea (https://github.com/sympy/sympy/pull/1696).

1) Given that there isn't an excepted convention, I think  the default should be the thing  that's easiest to understand.  Pre-calc students in high-school, who do not yet understand integration, will be really confused by  sum( f(x), (x,3,0)) != f(3) + f(2) + f(1) + f(0).

2)  We don't always want Sum(f(x),(x,a,b)) + Sum(f(x),(x,b,c)) == Sum(f(x),(x,a,c)).  It depends on context. 

3) Notation should make everything explicit, avoid ambiguity, and should not rely on implicit assumptions that are not easily discovered by a naive reader.  Examples of bad notation are einstein summation notation (not uniquely parsable) and stochastic differential equation notation (depends on the specific definition of stochastic integration used).   Summation notation is not a perfect parallel with integration -- there is no infinitessimal (dx) term that explicitly communicates some type of orientation is the summation.

I'd like to just be able to specify a summation in the form Sum(f(i) for i in W(n)) where W is an rather general collection of sets of integers parameterized by n, and not have to worry about the algebra system trying to tell me what  the signs of f(i) should be.