* 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 {
...
}