message: add missing I param to the Sub impl for CsVecBase
diff: @@ -1185,18 +1185,19 @@ where
  }
  }
  -impl<'a, 'b, N, IS1, DS1, IS2, DS2> Sub<&'b CsVecBase<IS2, DS2>>
  +impl<'a, 'b, N, I, IS1, DS1, IS2, DS2> Sub<&'b CsVecBase<IS2, DS2>>
  for &'a CsVecBase<IS1, DS1>
  where
  N: Copy + Num,
  - IS1: Deref<Target = [usize]>,
  + I: SpIndex,
  + IS1: Deref<Target = [I]>,
  DS1: Deref<Target = [N]>,
  - IS2: Deref<Target = [usize]>,
  + IS2: Deref<Target = [I]>,
  DS2: Deref<Target = [N]>,
  {
  - type Output = CsVec<N>;
  + type Output = CsVecI<N, I>;
  - fn sub(self, rhs: &CsVecBase<IS2, DS2>) -> CsVec<N> {
  + fn sub(self, rhs: &CsVecBase<IS2, DS2>) -> CsVecI<N, I> {
  binop::csvec_binop(self.view(), rhs.view(), |&x, &y| x - y).unwrap()
  }
  }
  