def _find(self, pair, default=None):
        """
        This is the environment implementation of
        :attr:`BaseKerning.find`. This must return an
        :ref:`type-int-float` or `default`.
        """
        from fontTools.ufoLib.kerning import lookupKerningValue
        font = self.font
        groups = font.groups
        return lookupKerningValue(pair, self, groups, fallback=default)