`impl Op<Quantity<...>> for Quantity<..>` is already complete. Add the following reference implementations: - [ ] `impl<'a, 'b> Op<&'a Quantity<...>> for &'b Quantity<..>` - [ ] `impl<'a> Op<&'a Quantity<...>> for Quantity<..>` - [ ] `impl<'a> Op<Quantity<...>> for &'a Quantity<..>`