not throw away information, so we can rule out `fun x -> []` as an
ideal unit. And units should not add more information than required,
so there's no obvious reason to prefer `fun x -> [x,x]`. In other
not throw away information, so we can rule out `fun x -> []` as an
ideal unit. And units should not add more information than required,
so there's no obvious reason to prefer `fun x -> [x,x]`. In other