'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):

    logger.debug(u'%s    missing    %r', distribution, requirement)
    self.missing.setdefault(distribution, []).append(requirement)
