sum(''(lhs(d5)),i,1,n);
> (%i1) x[i]+y[i]=0$
> (%i2) sum(rhs(%o1),i,1,n)
> (%o2) sigma_symbol rhs(0 = x[i] + y[i])
>
> How to force evaluation of rhs?
This is a bug in sum. (This may have been fixed in a more recent
version, but getting the semantics right is hard.)
The workaround is to use ''(...), e.g.
sum( ''( rhs(...) ), ...)
Note that that is two single-quotes in a row, not one doublequote.
-s