subclass like VaticanaLigatureEngraver. The property is lateron
read by the associated item class, such as VaticanaLigature.--jr
*/
-ADD_INTERFACE (Gregorian_ligature, "gregorian-ligature-interface",
+ADD_INTERFACE (Gregorian_ligature,
"A gregorian ligature",
"virga stropha inclinatum auctum descendens ascendens "
"oriscus quilisma deminutum cavum linea pes-or-flexa "