Returns all documented sub-modules in the module sorted
        alphabetically as a list of `pydoc.Module`.

def submodules(self):
        """
        Returns all documented sub-modules in the module sorted
        alphabetically as a list of `pydoc.Module`.
        """
        p = lambda o: isinstance(o, Module) and self._docfilter(o)
        return sorted(filter(p, self.doc.values()))