Checks all crossreferences found in the bibliography. If one can not
        be resolved from *this* Bibliography instance, a BrokenCrossReferences
        exception is raised.

def check_crossrefs(self):
        """
        Checks all crossreferences found in the bibliography. If one can not
        be resolved from *this* Bibliography instance, a BrokenCrossReferences
        exception is raised.
        """
        broken = []
        for value in self.values():
            crossref = value.get('crossref')
            if crossref is not None:
                if crossref not in self:
                    broken.append(value)
        if len(broken):
            raise exceptions.BrokenCrossReferences('One or more cross reference could not'
                    ' be resolved', broken)