def missing_input_files(self):
        """Make and return a dictionary of the missing input files.

        This returns a dictionary mapping
        filepath to list of `Link` that use the file as input.
        """
        ret_dict = OrderedDict()
        for link in self._links.values():
            link_dict = link.missing_input_files()
            for key, value in link_dict.items():
                try:
                    ret_dict[key] += value
                except KeyError:
                    ret_dict[key] = value
        return ret_dict