sub-trait, intersected-trait

[JeffreySarnoff]

Need to consider whether it makes sense to have subtraits or whether an
intersection would be better.

Without considering the interapplication of types, some uses differ. This arises from parentage, a subtrait has one parent and an intersected trait has one or more parents. Both can be used to specialize; subtrait specialization constrains the expression of its parent and ?intertrait specialization expresses the constraint of its parents.

[mauro3]

Note that traits can have several super-traits.