Question: In what year was the heavy metal band from Tronheim, Norway, who specializes in music that embodies a continuous move between explicit and implicit references to genres and strategies derived from cultural domains, formed?

<P0-S0> Darkthrone is a Norwegian heavy metal band. <P0-S1>  It formed in 1986 as a death metal band under the name Black Death. <P0-S2>  In 1991, the band embraced a black metal style influenced by Bathory and Celtic Frost and became one of the leading bands in the Norwegian black metal scene. <P0-S3>  Their first three black metal albums—"A Blaze in the Northern Sky", "Under a Funeral Moon" and "Transilvanian Hunger" (sometimes dubbed the "Unholy Trinity") — are considered the peak of the band's career and to be among the most influential albums in the genre. <P0-S4>  For most of this time, Darkthrone has been a duo of Nocturno Culto and Fenriz, who have sought to remain outside the music mainstream. <P0-S5>  Since 2006, their work has strayed from the traditional black metal style and incorporated more elements of traditional heavy metal, speed metal and punk rock, being likened to Motörhead. <SEP> <P1-S0>  Rooted in the idea of a cultural alternative, musical progressiveness embodies a continuous move between explicit and implicit references to genres and strategies derived from various cultural domains, such as European art music, Celtic folk, West Indian, or African. <P1-S1> Progressive music is music that subverts genre and expands stylistic boundaries outwards. <P1-S2>  The word "progressive" comes from the basic concept of "progress", which refers to development and growth by accumulation, and is often deployed in numerous music genres such as progressive country, progressive folk, progressive jazz, and (most significantly) progressive rock. <SEP> <P2-S0> Triosphere is a Norwegian heavy metal band from Trondheim, Norway, created in 2004 by singer and bassist Ida Haukland, and guitarist Marius Silver Bergesen. <P2-S1>  They are defined by their heavy metal sound emphasising their power metal-like melodies and choruses, while also featuring progressive elements. <P2-S2>  Bergesen is the composer of all instrumental parts, with Haukland acting as lyricist, and composer of the vocal parts. <SEP> <P3-S0> Kekal (sometimes stylized as KEKAL) is a heavy metal and electronic music band formed in 1995 in Jakarta, Indonesia. <P3-S1>  According to AllMusic, Kekal was one of the first heavy metal bands from Indonesia to make international inroads, and according to sociologist of heavy metal, Keith Kahn-Harris, was one of the few extreme metal bands from Southeast Asia to ever make more than a minimal impression on the global scene. <P3-S2>  Founded by two musicians known simply Yeris and Newbabe, the band underwent some shifts in lineup in its early years, but emerged with a consistent lineup of three key-members, guitarist/vocalist Jeff Arwadi, bassist Azhar Levi Sianturi, and guitarist Leo Setiawan. <P3-S3>  Frequently labeled as black metal, progressive metal, and avant-garde metal, Kekal plays a very diverse range of music styles within the frame of metal and rock, incorporating many other music genres such as ambient, electronic, jazz fusion, and progressive rock. <P3-S4>  Over the course of its career, Kekal has transitioned from a heavy metal-based style to a more experimental and electronic sound.