because the nature of what's being counted matters. I could write this

```rust
struct VelocitySensor<A, B> {
...
}
```
impl<A, B: Clock> Clock for VelocitySonsor {
....
}

impl<A: PosCounter, B> for VelocitySensor<A, B> {
...
}

impl VelocitySensor<A, B> where A: PosCounter, B: Clock {
   ...
}

```