(Unstable) Create a new overlay that acts like a chained map: Values
            missing in the overlay are copied from the source map. Both maps
            share the same meta entries.

            Entries that were copied from the source are called 'virtual'. You
            can not delete virtual keys, but overwrite them, which turns them
            into non-virtual entries. Setting keys on an overlay never affects
            its source, but may affect any number of child overlays.

            Other than collections.ChainMap or most other implementations, this
            approach does not resolve missing keys on demand, but instead
            actively copies all values from the source to the overlay and keeps
            track of virtual and non-virtual keys internally. This removes any
            lookup-overhead. Read-access is as fast as a build-in dict for both
            virtual and non-virtual keys.

            Changes are propagated recursively and depth-first. A failing
            on-change handler in an overlay stops the propagation of virtual
            values and may result in an partly updated tree. Take extra care
            here and make sure that on-change handlers never fail.

            Used by Route.config

def _make_overlay(self):
        """ (Unstable) Create a new overlay that acts like a chained map: Values
            missing in the overlay are copied from the source map. Both maps
            share the same meta entries.

            Entries that were copied from the source are called 'virtual'. You
            can not delete virtual keys, but overwrite them, which turns them
            into non-virtual entries. Setting keys on an overlay never affects
            its source, but may affect any number of child overlays.

            Other than collections.ChainMap or most other implementations, this
            approach does not resolve missing keys on demand, but instead
            actively copies all values from the source to the overlay and keeps
            track of virtual and non-virtual keys internally. This removes any
            lookup-overhead. Read-access is as fast as a build-in dict for both
            virtual and non-virtual keys.

            Changes are propagated recursively and depth-first. A failing
            on-change handler in an overlay stops the propagation of virtual
            values and may result in an partly updated tree. Take extra care
            here and make sure that on-change handlers never fail.

            Used by Route.config
        """
        # Cleanup dead references
        self._overlays[:] = [ref for ref in self._overlays if ref() is not None]

        overlay = ConfigDict()
        overlay._meta = self._meta
        overlay._source = self
        self._overlays.append(weakref.ref(overlay))
        for key in self:
            overlay._set_virtual(key, self[key])
        return overlay