Task: Question Answering
Input (English): Passage: By the late 1950s, Covent Garden was gradually abandoning its policy of productions in the vernacular; such singers as Maria Callas would not relearn their roles in English. This made it easier for Tucker to point up the difference between the two London opera companies. While Covent Garden engaged international stars, Sadler's Wells focused on young British and Commonwealth performers. Colin Davis was appointed musical director in succession to Gibson in 1961. The repertoire continued to mix familiar and unfamiliar operas. Novelties in Davis's time included Pizzetti's Murder in the Cathedral, Stravinsky's Oedipus rex, Richard Rodney Bennett's The Mines of Sulphur and more Janáček. Sadler's Wells's traditional policy of giving all operas in English continued, with only two exceptions: Oedipus rex, which was sung in Latin, and Monteverdi's L'Orfeo, sung in Italian, for reasons not clear to the press. In January 1962, the company gave its first Gilbert and Sullivan opera, Iolanthe, on the day on which the Savoy operas came out of copyright and the D'Oyly Carte monopoly ended.  The production was well received (it was successfully revived for many seasons until 1978) and was followed by a production of The Mikado in May of the same year.The Islington theatre was by now clearly too small to allow the company to achieve any further growth. A study conducted for the Arts Council reported that in the late 1960s the two Sadler's Wells companies comprised 278 salaried performers and 62 guest singers. The company had experience of playing in a large West End theatre, such as its 1958 sell-out production of The Merry Widow that had transferred to the 2,351-seat London Coliseum for a summer season. Ten years later, the lease of the Coliseum became available.  Stephen Arlen, who had succeeded Tucker as managing director, was the primary advocate for moving the company. After intense negotiations and fund-raising, a ten-year lease was signed in 1968.  One of the company's last productions at the Islington theatre was Wagner's The Mastersingers, conducted by Goodall in 1968, which 40 years later was described by Gramophone magazine as "legendary". The company left Sadler's Wells with a revival of the work with which it had re-opened the theatre in 1945, Peter Grimes. Its last performance at the Rosebery Avenue theatre was on 15 June 1968. 
Question: What production was successfully revived until 1978?
Output (English): Iolanthe.