Add a missing *requirement* for the given *distribution*.

        :type distribution: :class:`distutils2.database.InstalledDistribution`
                            or :class:`distutils2.database.EggInfoDistribution`
        :type requirement: ``str``

def add_missing(self, distribution, requirement):
        """
        Add a missing *requirement* for the given *distribution*.

        :type distribution: :class:`distutils2.database.InstalledDistribution`
                            or :class:`distutils2.database.EggInfoDistribution`
        :type requirement: ``str``
        """
        logger.debug('%s missing %r', distribution, requirement)
        self.missing.setdefault(distribution, []).append(requirement)