'Select the given item of the given item class
This method is called with the following guarantees:
- `name` is a valid selector class for this item
- `sel` is a valid selector for this item
- the `(name, sel)` tuple is not an "impossible" combination (e.g. a
selector is specified when `name` is not a containment object).
Return None if no such object exists'

@abc.abstractmethod
def _select(self, name, sel):

    pass
