Initially it was the intent of xxx_category to be deduced from the type signature.
As far as xxx_category defaults to the type itself, xxx_category specializations seems not needed.
If we decide to comeback to a xxx_category deduction, the xxx_traits will be not useful.
Otherwise if we don't want xxx_category deduction, xxx_category is not needed and the concept operations will be much simpler.
For the time been I replace xxx_category specialization by xxx_traits specialization.
Initially it was the intent of xxx_category to be deduced from the type signature. As far as xxx_category defaults to the type itself, xxx_category specializations seems not needed.
If we decide to comeback to a xxx_category deduction, the xxx_traits will be not useful. Otherwise if we don't want xxx_category deduction, xxx_category is not needed and the concept operations will be much simpler.
For the time been I replace xxx_category specialization by xxx_traits specialization.
We could remove the xxx_category later on.