message: Remove unnecessary mut binding.
diff: @@ -441,7 +441,7 @@ impl<E: Engine> Num<E> {
  ) -> Self
  {
  let newval = match (self.value, bit.get_value()) {
  - (Some(mut curval), Some(mut bval)) => {
  + (Some(mut curval), Some(bval)) => {
  if bval {
  curval.add_assign(&coeff);
  }
  