Gong are an international progressive rock band that incorporates elements of jazz and space rock into their musical style. The group was formed in Paris in 1967 by Australian musician Daevid Allen and English vocalist Gilli Smyth. Band members have included Didier Malherbe, Pip Pyle, Steve Hillage, Mike Howlett, Pierre Moerlen, Bill Laswell and Theo Travis. Others who have played on stage with Gong include Don Cherry, Chris Cutler, Bill Bruford, Brian Davison, Dave Stewart and Tatsuya Yoshida.

1972 saw the start of increasing line-up disruption for Gong. Laurie Allan left in April to be replaced by Mac Poole, then Charles Hayward and then Rob Tait, before returning again late in the year. Gilli Smyth left for a time, returning to Deia to look after she and Daevid Allen's baby son, and was replaced by Diane Stewart, who was the partner of Tait and the ex-wife of Graham Bond. Christian Tritsch moved to guitar and was replaced on bass by former Magma member Francis Moze, while the band's sound was expanded with the addition of synthesizer player Tim Blake.  In October they were one of the first acts to sign to Richard Branson's fledgling Virgin Records label, and in late December traveled to Virgin's Manor Studio in Oxfordshire, England, to record their third album, Flying Teapot. As they settled in, they were played a rough mix of Mike Oldfield's Tubular Bells, then already in production. Towards the end of their recording sessions they were joined by English guitarist Steve Hillage, whom they had met a few weeks earlier in France playing with Kevin Ayers, and who had replaced Oldfield in Ayers' band. He arrived too late to contribute much to the album, but would soon become a key component in the Gong sound.  Flying Teapot was released on 23 May 1973, the same day as Tubular Bells, and was the first installment of the Radio Gnome Invisible trilogy, which expounded upon the (previously only hinted at) Gong mythology developed by Allen. The second part, Angel's Egg, followed in December, now featuring the 'classic' rhythm section of Mike Howlett on bass and Pierre Moerlen on drums. In early 1974 Moerlen left to work with the French contemporary ensemble Les Percussions de Strasbourg and Smyth left to give birth to she and Allen's second son. They were replaced once again by Rob Tait and Diane Stewart, and the band moved from its French base at Pavillon du Hay to an English one at Middlefield Farm, near Witney, Oxfordshire. Moerlen, and later Smyth, returned in order to complete the trilogy with the album You, but by the time of its release, in October 1974, Moerlen was back with Les Percussions de Strasbourg and Smyth had settled permanently in Deia with her young sons. Prior to touring in support of You, Allen visited Smyth and the boys in Deia, while the rest of the band, including the departed Moerlen, recorded the basic tracks for Hillage's first solo album, Fish Rising. Moerlen was initially replaced in Gong by a succession of stand-ins (Chris Cutler, Laurie Allan and Bill Bruford) until former Nice and Refugee drummer Brian Davison took the job in early 1975. Smyth had already been replaced by Hillage's partner Miquette Giraudy.  In June 1974, Camembert Electrique was given a belated UK release by Virgin, priced at 59p, the price of a typical single at the time; a promotional gimmick which they had used before for Faust and would use again for a reggae compilation in 1976. These ultra-budget albums sold in large quantities because of the low price, but the pricing made them ineligible for placement on the album charts. The hope was that new fans would be encouraged to buy the groups' other albums at full price.

Answer the following question by taking a quote from the article: What is Radio Gnome Invisible trilogy?
Flying Teapot was released on 23 May 1973, the same day as Tubular Bells, and was the first installment of the Radio Gnome Invisible trilogy,