Until the 1950s, the acoustic, nylon-stringed classical guitar was the only type of guitar favored by classical, or art music composers. In the 1950s a few Contemporary classical music composers began to use the electric guitar in their compositions. Examples of such works include Luciano Berios Nones (Berio) (1954) Karlheinz Stockhausens Gruppen (Stockhausen) (1955–57); Donald Erbs String Trio (1966), Morton Feldmans The Possibility of a New Work for Electric Guitar (1966); George Crumbs Songs, Drones, and Refrains of Death (1968); Hans Werner Henzes Versuch über Schweine (1968); Francis Thornes Sonar Plexus (1968) and Liebesrock (1968–69), Michael Tippetts The Knot Garden (1965–70); Leonard Bernsteins Mass (Bernstein) (1971) and Slava! (1977); Louis Andriessens De Staat (1972–76); Helmut Lachenmanns Fassade, für grosses Orchester (1973, rev. 1987), Valery Gavrilin Anyuta (1982), Steve Reichs Electric Counterpoint (1987), Arvo Pärts Miserere (1989/92), György Kurtágs Grabstein für Stephan (1989), and countless works composed for the quintet of Ástor Piazzolla. Alfred Schnittke also used electric guitar in several works, like the "Requiem", "Concerto Grosso N°2" and "Symphony N°1".
Question: What were all the Classical composers that used electric guitars in their composition in 1968?
Answer: George Crumbs Songs | Francis Thornes Sonar Plexus | Liebesrock | Hans Werner Henzes