Vow::Mountable

The marker every mountable service shares. You don't include this directly: include Vow::Exportable (annotate the methods you want) or include Vow::Exportable::All (export every public method) both make a class a Mountable. Vow::Registry#mount / Registry.new restrict on this type, so either flavor mounts the same way, and Vow::Registry can stay ignorant of how a service decided what to export.

The concrete vow_install is generated by Vow::Exportable::Generated (the shared macro engine both mixins route through); this just names the contract the registry depends on.