This is the environment implementation of
        :meth:`BaseLayer.interpolate`.

        Subclasses may override this method.

def _interpolate(self, factor, minLayer, maxLayer, round=True,
                     suppressError=True):
        """
        This is the environment implementation of
        :meth:`BaseLayer.interpolate`.

        Subclasses may override this method.
        """
        for glyphName in self.keys():
            del self[glyphName]
        for glyphName in minLayer.keys():
            if glyphName not in maxLayer:
                continue
            minGlyph = minLayer[glyphName]
            maxGlyph = maxLayer[glyphName]
            dstGlyph = self.newGlyph(glyphName)
            dstGlyph.interpolate(factor, minGlyph, maxGlyph,
                                 round=round, suppressError=suppressError)