'Spec of the extendee of this package, or None if it is not an extension'

@property
def extendee_spec(self):

    if (not self.extendees):
        return None
    name = next(iter(self.extendees))
    for dep in self.spec.traverse(deptypes=('link', 'run')):
        if (name == dep.name):
            return dep
    if self.spec._concrete:
        return None
    else:
        (spec, kwargs) = self.extendees[name]
        return spec
