Columbia subject headings:
Biblioteca Nacional (Spain)--Catalogs.
Operas--Spain--Librettos--Bibliography--Catalogs.
Zarzuelas--Librettos--Bibliography--Catalogs.
Harvard subject headings:
Biblioteca Nacional (Spain)--Catalogs.
Librettos--Bibliography--Catalogs.
Operas--Spain--Librettos--Bibliography--Catalogs.
Theater--Spain--Bibliography--Catalogs.
Zarzuelas--Librettos--Bibliography--Catalogs.
Princeton subject headings: