Question: Who was a composer of an opera that Anton van Rooy appeared in?

Context paragraphs:
Parsifal: Parsifal (WWV 111) is an opera in three acts by German composer Richard Wagner.  It is loosely based on "Parzival" by Wolfram von Eschenbach, a 13th-century epic poem of the Arthurian knight Parzival (Percival) and his quest for the Holy Grail (12th century).

Charles van Rooy: Dr. Charles Joan Marie Adriaan van Rooy (23 January 1912, Rotterdam – 1 August 1996, Hilversum) had a political career as a mayor of various cities, minister and governor.  He was a member of the Katholieke Volkspartij (Catholic People's Party; KVP).

Susie Moloney: Susie Moloney (born February 27, 1962 in Winnipeg, Manitoba) is a Canadian author of horror fiction.  The film rights to her book, "A Dry Spell", were purchased by Cruise/Wagner Productions in 1997, for a reported seven figures.  Moloney was the first novelist to appear on the cover of Chatelaine magazine.  She was also featured on the cover of Scarlett Magazine.  Moloney has been compared to both Stephen King.  Moloney was the winner of the Michael Van Rooy Award for Genre Fiction.  Moloney splits her time between Canada and New York City.  Her husband is award-winning playwright Vern Thiessen.

Van Rooy: Van Rooy or Van Rooij is a Dutch toponymic surname.  "Rooij" was a local term for many towns ending with "rode" or "roij", like Nistelrode, Sint-Oedenrode, Stramproy and Wanroij.  This suffix itself means "a clearing made by men".  People with this surname include:

Norman Afzal Simons: Norman Avzal Simons (born 12 January 1967), also known as the "Station Strangler", is a South African rapist and serial killer who was convicted in 1995 on one count of murder and one count of kidnapping Elroy van Rooy, age 10.  He was sentenced to 35 years (25 for murder and 10 for kidnapping) in prison.

Anton van Rooy: Anton van Rooy (1 January 1870 – 28 November 1932) was a Dutch bass-baritone.  He had a voice of enormous proportions and is most remembered for his association with the music dramas of Richard Wagner, especially the "Ring Cycle", "The Mastersingers of Nuremberg" and "Parsifal".

Frans van Rooy: Frans van Rooy (born 3 July 1963) is a Dutch former professional footballer.

Kenneth Van Rooy: Kenneth Van Rooy (born October 8, 1993 in Turnhout) is a Belgian cyclist riding for Lotto-Soudal U23.  He will join Topsport Vlaanderen-Baloise in 2016.

Van Rooy (sheep): The Van Rooy, also known as the Van Rooy White Persian, is a breed of domestic sheep native to South Africa.  The Van Rooy was first developed in 1906 by (and named for)  J. C. van Rooy, a South African Senator and farmer in the Bethulie district.  The Van Rooy is cross between indigenous Ronderib Afrikaner sheep, Blackhead Persian, and Rambouillets.  It is a fat-tailed sheep and also a hair sheep, removing the need for shearing and crutching.  They are generally kept for meat production, and are very well-suited to arid climates.  Van Rooys are polled, have drooping ears, and are entirely white.  The breed is relatively rare, even in South Africa, but has also been exported to Namibia and Zimbabwe.

Australian White: Australian White is a large, white breed of Australian meat sheep which was developed for Australian conditions.  The Australian White sheep are a stabilised composite haired meat breed comprising White Dorper, Van Rooy, Poll Dorset and Texel genetics.

Supporting paragraphs: Anton van Rooy Parsifal
Answer: Richard Wagner