Like Velocity or Acceleration, so I could define methods that could take a Velocity in any set of units
I had originally tried designing a concept of "abstract quantity" (duration, length, velocity, etc) explicitly into the type system, but it turned out to be far more flexible and elegant to have abstract quantities emerge naturally from the definitions of canonical form and convertability. So two unit types are of the same abstract quantity if and only if they are convertable, and also if and only if their canonical forms are the same.