def _match_overlay(self, raster, overlay_spec):
        """
        Given a raster or input overlay, generate a list of matched
        elements (None if no match) and corresponding tuple of match
        strength values.
        """
        ordering = [None]*len(overlay_spec) # Elements to overlay
        strengths = [0]*len(overlay_spec)   # Match strengths

        elements = raster.values() if isinstance(raster, Overlay) else [raster]

        for el in elements:
            for pos in range(len(overlay_spec)):
                strength = self._match(el, overlay_spec[pos])
                if strength is None:               continue  # No match
                elif (strength <= strengths[pos]): continue  # Weaker match
                else:                                        # Stronger match
                    ordering[pos] = el
                    strengths[pos] = strength
        return ordering, strengths